The nitric-oxide (NO)-cyclic-guanosine-monophosphate (cGMP) pathway plays a key role in penile erection. Erectile dysfunction (ED) is a complication in male diabetic patients that impacts their quality of 1ife. Recently, Yidiyin, a Chinese herbal decoction, is used to treat diabetic ED, but convincing evidence is lacking, and the potential mechanisms remain uncertain. In the study, diabetic ED patients had low scores on international index of erectile function-5 (IIEF-5), and administration of Yidiyin and hypoglycemic drugs for 16 weeks ameliorated patients' scores on IIEF-5 more than the hypoglycemic drug alone. Moreover, streptozotocin-induced diabetes severely impaired rats' erectile function and the activity of the NO-cGMP pathway in the corpora cavernosum, and treatment with Yidiyin for 4 weeks obviously increased the rats' erectile function, remarkably enhanced the activity of nitric oxide synthase (NOS), and elevated the contents of NO and cGMP. Our findings indicate that Yidiyin improves diabetic ED probably by enhancing the NO-cGMP pathway.