Background: To improve the athletic ability and muscle mass, anabolic androgenic steroids are abused by athletes. Little is known about how these compounds affect the fine structures of the testis. This study aimed to identify the changes in the fine structure of testis following administration of nandrolone decanoate. Methods: Twenty five Sprague-Dawley rats were randomly divided into two experimental, two vehicle, and one control groups. Experimental groups were treated by 3 or 10 mg/kg/wk intramuscular injection of nandrolone decanoate. The vehicle groups were treated by the same amount of peanut oil, for 14 weeks. One week after the last injection, the rats were sacrificed and their testes were prepared for transmission electron microscopy study. Results: The cells in the interstitial space of the experimental rats were considerably degenerated. The basement membrane was thick and the diameter of seminiferous tubule was reduced. Several degenerated Sertoli cells and apoptotic germ cells were considerably observed in the experimental rats. Conclusion: The results of this study show that fine structure of the testis is affected by nandrolone decanoate.