The activation of cavernous local renin-angiotensin system has an important role in pathogenesis of diabetic erectile dysfunction (ED). In our primary study, we found that angiotensin Type 1 receptor blocker improved the erectile function of diabetic rats. Therefore we explored the losartan in clinical treatment for diabetic patients suffering with ED. A total of 124 diabetic patients with ED were included in this study and treated with losartan or tadalafil or losartan plus tadalafil or watch for waiting as control for 12 weeks. Erectile function was assessed by International Index of Erectile Function (IIEF-5) questionnaire, the percentage of positive responses to sexual encounter profile questions 2 (SEP2), 3 (SEP3) and the global assessment question (GAQ). Losartan or tadalafil or losartan plus tadalafil significantly improved the mean IIEF-5 scores, the percentage of successful penetrations (SEP2), the successful intercourse completions (SEP3) and GAQ (P<0.05). The combination of losartan and tadalafil is more effective than the single-use of losartan or tadalafil (P<0.05). The patients with moderate and mild ED had better response rates to losartan than patients with severe ED. This is the first clinical trial in losartan therapy on diabetic patients suffering from ED. Losartan seems to be effective and well-tolerated in diabetic ED patients, especially for mild to moderate ones. The combination therapy of losartan and tadalafil appeared to be more effective than monotherapy.