Introduction: Using grape seed oil in the diet can play an important role in the health of consumers, due to its richness of essential fatty acids and antioxidant compounds such as tocopherols and polyphenols. However, the oxidative stability of this oil is relatively low and mixing it with more stable oils such as sesame oil can be a good way to improve its oxidative stability. However, during the heating of vegetable oils, some of their physical and chemical properties change, such as: increasing the number of acidic and peroxide, reducing the oxidative stability, changing the type and amount of pigments, and eventually forming a dimmer and Polymers that increase the viscosity of oils (Casal et al., 2010). The effect of frying heat on the composition and chemical properties of various vegetable oils such as olive oil (Romano et al., 2012) and sunflower oil (Norstand et al., 2006) have been tested by various researchers. However, no research has ever been carried out on the effect of frying on the quality characteristics of a mixture of sesame oil and grape seed oil. Accordingly, after obtaining an optimum formulation of sesame and grape seed oil mixture in the initial research (Khakbaz Heshmati et al., 2019), in this study, possible changes in some of the qualitative properties of optimal formulation during the frying process were investigated and also, an optimal sample was compared with commercial frying oil to evaluate the effect of frying on some quality properties of a mixture of sesame oil and grape seed oil, to produce oily products with beneficial nutritional properties without synthetic anti-oxidants for frying. Material and methods: In the preliminary research carried out by Khakbaz Heshmati et al. (2019), the samples obtained from mixing different proportions of sesame oil (0, 25, 50, 75 and 100%) with grape seed oil (100, 75, 50, 25 and 0%) was prepared and evaluated in terms of peroxide number, iodine number, fatty acid profile, oxidative stability, total phenol and antioxidant activity, the results of this study showed that with increasing the ratio of sesame oil from 25 to 75%, the linoleic acid content of the samples decreased; while oleic, palmitic and stearic fatty acids increased in combined samples. Increasing the proportion of sesame oil in the mixture, reduced the iodine number and peroxide index of the samples. The addition of sesame oil in different ratios to grape seed oil, increased the number of phenolic compounds and antioxidant strength, resulting in the oxidative stability of the mixed samples. According to the results of the evaluation of chemical and antioxidant properties and also due to the low price of the product, the formulation of 25% grape seed oil + 75% sesame oil was selected as the optimal formulation for different mixing percentages of grape seed and sesame oils (Khakbaz Heshmati et al., 2019). To proceed with this study, sesame oil and grape seed oil of 200 grams (w/w) were poured in a laboratory container and homogeneously mixed with a magnetic stirrer (at ambient temperature) to form a uniform mixture. In the next step, to test the oil quality changes during the frying process, four samples of 150 grams of optimum oil formulation were prepared without any additives and were poured into glass containers. Samples were heated in an oven at 160 ° C and at 0 ° C (control sample), for 30, 60 and 90 minutes. After cooling in the desiccator, the samples were stored at-18° C until the test. Also, a commercial oil (a combination of sunflower, soybean, palm olein, palm super olein, and canola) used as the control sample in this study. It was purchased immediately after production and kept until the research was carried out at-18 ° C. In this research, the thermal stability of the mixture of sesame oil (75%) and grape seed oil (25%) as the Optimized formulation, was investigated for 90 minutes of frying process at 180° C and compared with the frying oil. During the frying process, at 30 minutes intervals, the main oil qualitative indicators including acid number, peroxide value, p-anisidine value, phenolic compounds, and polar components were measured. Results and discussion: In this study, a mixture of sesame oil and grape seed oil was heated at 180° C and, the quality factors of the oil were investigated for 90 minutes. The results showed that by the increasing heating time acid value of the mixture of sesame oil and grape seed oil increased significantly, which resulted in hydrolysis and release of fatty acids. By increasing heating time up to 60 minutes due to oxidation of peroxides, peroxide value increased significantly. However, by the continuation of the heating process, the initial oxidation products were unstable and, by the decomposition of these products, the peroxide value was reduced. By the decomposition of peroxides, the anisidine value increased significantly. With the increasing heating time, the oxidation of phenolic compounds increased and, the number of phenolic compounds decreasing. Comparison of the acid values and peroxide value of the mixture of sesame oil and grape seed oil with the permitted values of the Iranian national standard showed that these values were not within the permitted range while the number of polar compounds was within the allowable range. As a result, it was noted that the mixture of sesame oil and grape seed oil, although suitable for short-term heating and cooking. But it did not gain usability as a frying oil. It is not an applicable oil for long-term heating and frying. The results of specific tests of frying oils showed that, with increasing frying time, the acid number, panisidine value, and polar components were increased but peroxide value increased only for the first 60 minutes of the process and then decreased. Conclusion: Comparison of the obtained values for different factors of sesame and grape seed oil with the national Iranian standard limits showed that the parameters of acid number and peroxide value were more than the national standard of Iran, but the content of polar compounds was within the permissible national standard of Iran. Finally, the mixture of sesame oil and grape seed oil is not suitable for long term heating and frying.