Background: Iron oxide nanoparticles are commonly used for various purposes, such as biomedical, medical, and cosmetic services and research. However, there is a little information about the effects of the nanoparticles on human health. The current investigation was conducted to evaluate the adverse effects of iron oxide nanoparticles (FeNP) on the reproductive organs of mice, such as the testicular tissue and sperm cells. Methods: Twenty-eight male NMRI mice were randomly divided in four groups (N=7). The control group received only a regular diet. The experimental groups were administered FeNP in doses of 50, 150 and 300 mg/Kg intraperitoneally (IP), over four days. Epididymal sperm parameters, such as sperm number and motility were assessed by computer-assisted sperm analysis (CASA). Stereological analysis was also conducted on the histological sections. Results: The results demonstrated that FeNP (300 mg/Kg/day) caused a significant decrease in the sperm parameters, such as motility, spermatogonia, primary spermatocytes, spermatid, Sertoli, Leydig cells, total length of seminiferous tubules, and testicular interstitial tissue volumes. Conclusion: In summary, FeNP affected several reproductive tissue and cellular parameters at the administered dosage. Further research is required to examine the mechanism of action of FeNP the mice reproductive system.