Objective: The aim of this study was to evaluate the effect of cyanidin-3-O-β-D-glucopyranoside (C3G) concentrated materials from mulberry fruit on improvement and protection of erectile function. Materials and Methods: Sprague-Dawley rats (12 weeks old) were divided into three groups (n = 12 in each): normal control, diabetes mellitus (DM), and DM with C3G concentrated material treatment (DM + C3G). DM and DM + C3G group rats received a single injection of streptozotocin (50 mg/kg), and 4 weeks after induction of diabetes, the DM + C3G group rats were treated with daily concentrated material treatment (10 mg/kg) dissolved in water for 8 weeks. After 12 weeks of streptozotocin injections, the rats in each group underwent intracavernosal pressure measurement and then the corporal tissues were sampled. Results: The DM group rats showed markedly lower erectile parameters than those in the control group, whereas rats in the DM + C3G group showed improved erectile function by minimizing corporal apoptosis and increasing the expression of endothelial nitric oxide synthase (NOS) and neuronal NOS protein. A significant increase in 8-hydroxy-2-deoxyguanosine (8-OHdG) was shown in the DM group compared with the normal group. However, in the DM + C3G group, 8-OHdG was statistically significantly reduced compared with the DM group. Conclusions: The current study is the first to suggest that C3G concentrated materials may have a potency to improve and protect erectile function under conditions of diabetes-induced oxidative stress.