Purpose: To compare monotherapy with tadalafil or tamsulosin and their combination therapy in men with benign prostatic hyperplasia and erectile dysfunction by comparing IPSS score, prostate volume and Qmax and some other outcomes. Materials and Methods: This randomized, single-blind, paralleled group clinical trial was done in 2013 on patients who had referred to our hospital in Tehran. All patients with lower urinary tract symptoms, benign prostatic hyperplasia and any grade of erectile dysfunction were recruited. They were randomly divided into three groups (61 participants in each group): Group A received 20 mg/daily tadalafil; Group B received 0.4 mg/daily tamsulosin; Group C receieved a combination of 0.4 mg/daily tamsulosin and 20 mg/daily tadalafil. Primary outcomes were prostate volume, prostate specific antigen, post-void residual volume, IPSS score, LUTS severity, Qmax, IIEF and erectile dysfunction severity and secondary outcome was complications. Results: The mean SD of ultrasonographic prostate volume was 61.4 +/- 15.1 mL and prostate specific antigen level was 2.4 +/- 1.9 ng/dl. Post-void residual level was significantly different before and after the treatment, except for group A. Also, this group had no meaningful difference compared to the other groups in this regard (P >0.05). There were significant differences between pre-and post-treatment international prostate symptom scores in each group (P < 0.05). Conclusion: Combination of tamsulosin and tadalafil can improve international prostate symptom scores, international index of erectile function questionnaire scores and Qmax in patients with lower urinary tract symptoms and benign prostatic hyperplasia to more degrees than their separate use. This combination is recommended because of its synergistic effects, well toleration and safety.