Background Chronic hyperglycemia is a characteristic of diabetes mellitus (DM). Long-lasting hyperglycemia can generate oxidative stress and reactive oxygen species. The effect of this condition on sperm quality and spermatogenesis leads to male infertility and reproductive dysfunction. Objectives To investigate changes in sperm quality, morphology of testicular structure, and stage of development of seminiferous tubules in a streptozotocin (STZ)-induced rat model of type 1 DM. Methods We divided 15 male Sprague Dawley rats into 2 groups. DM was induced in 7 rats using STZ (60 mg/kg intraperitoneally), while the other 8 were treated with citrate buffer as a vehicle control group. Rat semen was collected for quality measurements including motility, normal morphology, and concentration. Morphological changes in testicular structure and stage of development of seminiferous tubules were investigated by histology with hematoxylin and eosin (HE) staining. Results Significant decreases in all parameters of sperm quality and testicular weight were found in rats with induced DM. Moreover, abnormal morphology of seminiferous tubules including separation of the germinal epithelium, vacuolization, luminal sloughing of germ cells, and tubular atrophy was increased significantly in these rats, while the proportion of their seminiferous tubules at an early stage of development was significantly higher, but was dramatically decreased in the late stage of development when compared with that in vehicle-treated control rats. Conclusions DM has adverse effects on sperm quality, testicular structure, and development of seminiferous tubules. These findings may reflect the male infertility and reproductive dysfunction seen in patients with type 1 DM.