Aim: To evaluate the efficacy and safety of SK3530, a newly developed type 5 phosphodiesterase inhibitor (PDE5I), in Korean men with erectile dysfunction (ED). Methods: A total of 119 patients were randomized at 10 centers in Korea to receive either SK3530 (50, 100, or 150 mg; n = 89) or placebo (n = 30) taken 1 h before anticipated sexual activity for an 8-week period. The patients were evaluated at baseline and 4 and 8 weeks after beginning therapy. Efficacy was assessed using the International Index of Erectile Function (IIEF), Sexual Encounter Profile (SEP), and the Global Assessment Question (GAQ). Safety was analyzed by adverse events, laboratory values and vital signs. Results: At the end of the study, all the primary and secondary efficacy end-points were statistically significantly improved by SK3530 compared with placebo (P < 0.05). Of the 89 patients in the treatment arm, 36 (42.3 %) achieved normal erectile mfunction after treatment, including six patients with severe ED. Treatment-related adverse events occurred in 32 patients.The most common adverse events were flushing, headache, dizziness and eye redness (10.9%, 7.6%, 2.5% and 2.5%, respectively), and most were mild. Only two patients discontinued treatment during the study period because of adverse events. Conclusion: The results of our phase Ⅱ study have confirmed the efficacy and safety of SK3530 in a broad population of men with ED of various etiologies and severity. The optimal doses in terms of efficacy and safety were determined to be 50 mg and 100 mg, respectively.