Nonylphenols (NP) are used in manufacturing antioxidants and lubricating oil additives, which are considered to be as potent endocrine disruptor. The aim of this study was to examine the effects of NP on reproductive system in adult male mice. Mice were divided into 4 groups; (1) tap water (CON group), (2) 50 μg/L (NP50), (3) 500 μg/L (NP500) and (4) 5000 μg/L (NP5000) of NP via the drinking water for 4 weeks. Mice were sacrificed and the reproductive organ weights were measured. The caudal epididymal sperms were count to assess the toxicity on germ cells. Histopathological changes of tissues were observed by using hematoxylin/eosin staining. The weights of testis in NP5000 group were significantly lighter than those in CON group (104.9±2.9 mg vs 90.7±5.1 mg, p<0.05). And weights of epididymis significantly increased in NP500 group (44.2±2.6 mg vs 54.42±3.44 mg, p<0.05). As concentration of NP increased, the number of sperms significantly decreased (NP50 and NP500, p<0.01; NP5000, p<0.001). In histopathological analyses, the sperms in seminiferous tubules showed a concentration-dependent decrease in mice treated with NP. In epididymis, treatment with NP resulted in empty space and the reduced sperm numbers in dose-dependent manner. Our results confirmed the dose-dependent decrease in the number of sperm and histopathological abnormality of testis and epididymis in mice exposed from 50 μg/L to 5000 μg/L of NP for 4 weeks. The present study suggests that oral exposure of NP might have negative effects on reproductive function, particularly on germ cells, in adult male mice.