Of different types of bio-compatible nano‌particles, iron oxide nanoparticle has attracted attention for its applications especially in medicine and magnetic resonance imaging (MRI). We decided to study histological changes in testis and spermatogenesis in mature male mice that are affected by ironoxide nanoparticles (Fe₂O₃) during fetal period. In this study, the mice were divided into three experimental and two control and sham groups. Iron oxide at do-ses of 10, 30, 50 mg/kg were injected into pregnant mice during 10th, 12th and 14th days of the critical period of fetus testis development. After the maturation of the mice which were born, slices with the thickness of 0.06 µm were prep-ared from adult male testis and epididymis, stained with hematoxylin and eosin. The cells were counted, then the obta-ined data was analyzed with one-way ANONA and Tukey testes. The results showed that the number of epididymis sp-erms at the doses of 10 and 30 mg/kg, testis weight at the dose of 30 mg/kg, primary spermatocytes at the dose 10mg/kg had increased, while the testis diameter in all the three experimental groups and the volume of the testis at the dose of 50 mg/kg had decreased. Disorganization and vacuolization were observed at high doses. Passing through the cell me-mbrane, and considering its oxidation and reduction potentials, it was observed that Iron oxide nanoparticle acts as anti-oxidant at low doses and shows toxicity at high doses. The fact obtained in the paper reflects the dual potential of the ir-on oxide nanoparticle.