Introduction. One of the diabetes complications in men is infertility. C-peptide and Nicotinamide have antidiabetic and antioxidant activity. So, the aim of this study was to investigate the effects of these drugs on male infertility caused by diabetes. Methods. In this experimental study 40 adult male NMRI mice (20-25g) were obtained and randomly divided into 5groups:. controls, diabetes model, diabetes+C-peptide, diabetes+Nicotinamide, diabetes+C-peptide+Nicotinamide. Diabetes induced was confirmed 3 days after administration of a single dose of streptozotocin (STZ) (65mg/kg) 15 min after an intraperitoneal injection of Nicotinamide (120mg/kg). Then C-peptide (25nmol/kg) and Nicotinamide (100mg/kg) were injected for 28 days. 24h after the last drugs injection serum samples, testes and cauda epididymis of animals were removed for hormonal, testis morphology and sperm count assessment. Results. Diabetes induced could decrease serum testosterone level and sperm count significantly (p<0.001, p<0.05 respectively). Serum LH, testosterone levels and sperm count increased in Diabetes+Cpeptide+Nicotinamide versus diabetes group (p<0.05, p<0.001, p<0.01 respectively). Also administration of Nicotinamide alone showed an increase in serum testosterone (p<0.001). Conclusion. The results of this study demonstrated that combined administration of Nicotinamide and C-peptide improved diabetes induced male reproductive disorders by enhanced serum LH, testosterone levels and sperm count in diabetic mice.