Objective: In recent years, numerous reports indicate of the negative effects of electromagnetic fields on biological system. To evaluate the effects of 50Hz electromagnetic fields on parameters of oxidative stress in pregnant rats and its effects on ovarian tissue in F1 generation rats during puberty. Methods: In treatment group pregnant Wistar rats were exposed 3mT EMF for 21 days, 4 hours per day. Pregnant rats under same condition of treatment group, but off the field as a Sham group intended and pregnant rats were used as control in the room. After delivery the blood sample of mothers for biochemical analyze of MDA and SOD provided. For investigation of ovarian tissue pups, they were kept until maturity. Then adult female ovary of F1 generation were removed, fixed and prepared for light microscopy studies. Results: Biochemical analysis showed that MDA was significantly increased in the treated group in comparing with the controls, but no significant differences in SOD levels were observed between the groups. Microscopic results of the follicles F1 generation of treated group, in comparison with the control and Sham, showed that granulosa cells have interspaced from the basement membrane and in this group narrow and irregular in zona pellucida, vacuolization in ooplasm, detachment of granulosa cells was observed. Conclusion: The results suggest that pregnant maternal exposure in magnetic field cause of increases some of parameters of oxidative stress and also adverse effect on ovarian follicles in F1 generation during maturation and may impact on fertility.