The aim of this study was to investigate the protective effects of trimetazidine (TMZ), as an antioxidant agent, on streptozotocin (STZ)-induced diabetic rats. A total of 50 male Sprague Dawley rats were randomly classified into five groups as follows: Group 1 (control), Group 2 (STZ-induced diabetic rats), Group 3 (STZ-induced diabetic rats treated orally with 1cc/day isotonic saline), Group 4 (diabetic rats treated orally with 10mg/kg/day TMZ) and Group 5 (diabetic rats treated orally with 20mg/kg/day TMZ). After 8weeks, orchiectomy was carried out. Histopathological and electron microscopic examinations were performed in all groups. In groups 1 and 5, the structural and ultra-structural findings of the testicular tissue and spermatogenesis were found normal. In groups 2, 3 and 4, similar results were obtained in terms of the impaired testicular architecture and degeneration of spermatogenesis. The administration of an optimal dose of TMZ protects against the harmful effects of diabetes mellitus on spermatogenesis in rats. TMZ therapy can be used to maintain normal spermatogenesis in diabetic rats.