Objective To investigate the clinical significance of serum testosterone level in senile male patients with diabetic nephropathy (DN). Methods A total of 80 male diabetic patients over 75 years old admitted in our department from January 2013 to June 2014 were enrolled in this study. Thirty-four of them were diagnosed as DN and then assigned as DN group, and the other 46 without renal disease as non-diabetic nephropathy (NDN) group. Another 20 age-matched males without diabetes taking physical examination during same period served as controls. Their serum levels of testosterone, cystatin C (CysC) and urine microalbumin were detected. Their kidney ultrasound images were collected. The results were compared between the DN and NDN groups. Results Compared with control group, the serum level of testosterone was lower in diabetic male patients (P＜0.01). The serum level of testosterone was markedly lower [(13.18±3.29) vs (22.07±7.53)nmol/L, P＜0.01], but that of CysC was higher [(2.61±0.96) vs (1.09±0.34)mg/L, P＜0.01] in DN group than in NDN group. Serum level testosterone was negatively correlated with CysC (r＝-0.658，P＜0.01). Conclusion In the senile male diabetic patients, those with DN have lower serum testosterone level than those without, which is negatively correlated with CysC level. Serum testosterone level may be helpful to develop early intervention strategies to improve prognosis.