Introduction: Erectile dysfunction is a common condition among diabetic men. Many treatments are now available with variable responses. Aim: This study aimed to evaluate the effect of daily oral L-arginine plus tadalafil in diabetic patients with mild to moderate erectile dysfunction. Methods: A double-blinded, randomized, controlled clinical trial was conducted with 108 diabetic male patients. Each patient was assessed by medical and sexual histories, International Index of Erectile Function 5-item questionnaires, pharmaco-penile duplex ultrasonography, and serum testosterone level. Main Outcome Measure: Improvement in International Index of Erectile Function 5-item, serum testosterone level and pharmaco-penile duplex ultrasonography. Results: Erectile functions were significantly improved in all patients after treatment as compared with baseline and placebo (P .001). Patients who received both drugs showed significant improvement compared to those treated with single drugs, as assessed by International Index of Erectile Function scores and total testosterone (P .001). Pharmaco-penile ultrasound duplex results showed non-significant differences among patients treated with both drugs and those with each drug alone. Conclusion: Daily use of L-arginine with tadalafil significantly increased the International Index of Erectile Function scores and total testosterone levels as compared to each drug alone in diabetic patients with erectile dysfunction. No differences were found based on pharmaco-penile duplex findings