Aim and Background: Carotenoids are one of the most abundant pigments in nature and an essential compound of photosynthesis apparatus. In addition, carotenoids play a significant role in human and animal diet, because they are precursors for vitamin A and act as antioxidants. Thus, increasing carotenoid levels in plants, especially cereals as the main food for human and animals, could have a dramatic impact on improving plant yield and nutritional value. Among several enzymes involved in carotenoid biosynthesis pathway, PSY has been known as a key enzyme. So far, three PSY genes have been identified in maize; however, all related enzymes do not show similar activity in all tissues. Considering existing of three PSY genes, whether over-expression of this enzyme can still increase carotenoid biosynthesis levels in maize, is largely unknown.Materials and methods: Tissue culture and transformation was carried out using the method developed by Ahmadabadi et al. (1). Total DNA was extracted from maize leaves according to Doyle & Doyle method (5). To confirm transgene presence in regenerated transgenic plants, PCR reaction was performed using specific primers. Pigment content of transgenic plants was analyzed by HPLC method.Results: In this study, through expression of a foreign PSY gene in maize, we showed that maize include a high potential to increase carotenoid levels via over-expression of PSY, and thus, the PSY gene is a rate-limiting enzyme in carotenoid biosynthesis pathway in maize. It should be mentioned that transgenic plants were similar to wild-type plants in terms of growth, development and yield, while the general carotenoid levels was significantly increased. In particle, the level of beta-carotene as a precursor of vitamin A was improved considerably.Conclusion: Our results show that maize has a high potential for increasing carotenoid levels, especially beta-carotene, that can have a significant role in maize breeding programs in order to improve its nutritional value without any negative effect on yield quantity.