One of the most important defense systems in fish to keep homeostasis is antioxidant defense system. To assay antioxidant enzymes in ovarian maturation process in cultured Sterlet sturgeon Acipenser ruthenus females, 24 fish were identified in four groups: previtellogenic, vitellogenic, post-vitellogenic and atresia (6 in each group) and blood samples were taken. Maturity stages were identified on the basis of histological criteria and direct observation. The activity of catalase (CAT), superoxide dismutase (SOD(, glutathione peroxidase (GPX) and malondialdehyde (MDA) content in blood was investigated. The results showed CAT and GPX activity decreased in the vitellogenic stage in compare to previtellogenic (P <0.05), but showed an increase till atresia (P <0.05). SOD activity in different stages was not significantly different (P> 0.05). MDA concentration in different groups did not show significant changes (P> 0.05). The results of this study showed nevertheless the metabolism increased in the process of sexual maturation, the antioxidant defense is could effectively protect fish against these changes.