Diabetes-induced ED correlates with diabetes duration and glycemic control. This study evaluated the effect of glycemic control start time on erectile function in streptozotocin-induced diabetic rats. Rats were divided into normal controls (C); untreated diabetic rats (DM); and rats treated after 7 weeks (7W), and 10 weeks (10W) from DM. Treated diabetic rats received a timed daily injection of insulin. After 14 weeks of lab-controlled diabetes, experiments were performed. Group DM showed the ratio of intracavernosal pressure, significantly lower than other groups (10W vs DM; P<0.001). Groups 7W and 10W responded similarly, but did not recover to normal level (group C vs 7W; P<0.001). The percentage of α-smooth muscle actin increased more with earlier start times, and group DM's percentages decreased significantly (group 10W vs DM; P<0.001). Apoptosis recovered significantly only in group 7W, comparable to group C. As start times became earlier, for all molecules (eNOS, Akt, MYPT1 and PECAM-1), treatment groups' results neared those of group C. In conclusion, erectile function of diabetic rats recovered closer to normal controls if diabetic treatment started earlier. And the level of glycemic control was expected to be more important than the start time of diabetic treatment.