Background: Oxidative stress, defined as a disturbance in the balance between the production of reactive oxygen species (free radicals) and antioxidant defenses, is discussed in relation to its possible role in the production of tissue damage in diabetes mellitus. Regarding the prevalence of gestational diabetes mellitus in recent decades and the results of some studies about the effect of nutritional anomalies and oxidative stress, as well as insufficiency of studies on its occurrence, this study aimed to compare of oxidative stress indices in women with and without gestational diabetes mellitus. Methods: In a case-control study conducted in Isfahan Health Center No. 2, Isfahan, Iran, in 2017-2018, 37 women with gestational diabetes mellitus and 53 women without it mellitus were selected using convenient sampling method. Oxidative stress indices including glutathione (GSH), total antioxidant capacity, superoxide dismutase, non-esterified fatty acids (NEFA), catalase, and malondialdehyde were measured and compared between the two groups. Findings: The mean total antioxidant capacity was 441. 6 ± 63 and 562. 5 ± 44. 9 μ mol/l in women with and without gestational diabetes mellitus, respectively (P = 0. 011). The mean catalase level in the two groups was 0. 32 ± 1. 12 and 1. 79 ± 0. 59 U/l, respectively, and the difference between the two groups was statistically significant (P = 0. 024), but the other factors were not significantly different between the women with and without gestational diabetes mellitus. Conclusion: The levels of some oxidative stress markers such as total antioxidant capacity and catalase were significantly higher in women with gestational diabetes mellitus. Therefore, oxidative stress seems to play a role in the pathogenesis of gestational diabetes.