The purpose of this study was to investigate the effect of cyclophosphamide on the mouse seminiferous tubules by transmission electron microscopy at different groups: a control group, a group treated one time a week, and a group treated two times a week. Cyclophosphamide was injected in the intraperitoneal at a dosage of level 200㎎/ ㎏. The results obtained were as follows. 1. In the group of one time a week, pyknotic body and large vesicles were observed in cytoplasm of spermatogonium of seminiferous tubules, and in the intercellular space between spermatocytes, respectively. 2. In the group of two times a week, nucleus envelope in the spermatid was disrupted partially, observed vesicles in the nucleoplasm of spermatid, and separated or disrupted inner and outer membranes of mitochondria in the Sertoli cells.