Aim: In this study, it was aimed to determine the effect of CoQ<inf>10</inf> on testes in streptozotocin (STZ) induced diabetic rats. Materials and Methods: In this study, thirty-four adult male Wistar Albino rats were used. The animals were divided into five groups: Control group (K), oil group (Y; daily intraperitoneal injection of 0,3 ml corn oil), CoQ<inf>10</inf> group (CoQ<inf>10</inf>; daily intraperitoneal injection of 10 mg/kg CoQ<inf>10</inf>), Diabetes group (D; subcutan injection of 40 mg/kg STZ, single daily dose for two days), and Diabetes+ CoQ<inf>10</inf> group (D+CoQ<inf>10</inf>; Diabetic rats was injected daily intraperitoneally 10 mg/kg CoQ<inf>10</inf>). At the end of fourth week, tissue samples were obtained from sacrified animals. Testes were weighed, then fixed in Bouin's solution and processed by means of routine histological techniques. Sections were stained with H&E staining method to determine the general histological structure. Also, PCNA reactivity was determined by immunohistochemical method. Results: The mean body weight was significantly decreased in diabetes and diabetes+ CoQ<inf>10</inf> groups when compared to control, oil and CoQ<inf>10</inf> groups but the gonadosomatic index (GSI) was found higher in diabetes groups (p<0.05). Mean seminiferous tubular diameter (STD), height of germinal epithelium (GEH) and PCNA index were significantly reduced in diabetes groups in comparison with control, oil, CoQ<inf>10</inf> and diabetes+CoQ<inf>10</inf> groups (p<0.05). In diabetes+CoQ<inf>10</inf> group showed significantly increase in STD, GEH and PCNA index compared to diabetes group (p<0.05). Conclusion: In conclusion, CoQ<inf>10</inf> play a role in reducing the cellular damages caused by diabetes.