The present study was designed to evaluate the relaxant effect of Epimedium koreanum (E. koreanum), fractions of E. koreanum (n-hexane, ethyl acetate, and n-butanol) and major compound of E. koreanum on rabbit penile corpus cavernosum smooth muscle (PCCSM). Rabbit PCCSM was treated with E. koreanum extract (EKE), fractions, or major compound, icariin, after pre-incubation with 10 μM phenylephrine (Phe). All of EKE, fractions, and icariin relaxed the PCCSM in a concentration-dependent manner. The interaction of icariin with phosphodiesterase-5 (PDE-5) inhibitor, udenafil (UDE), was also examined on PCCSM contractility. Interestingly, the treatment of icariin markedly increased the UDE-induced relaxation of the PCCSM. HPLC analysis was also performed to investigate the extraction method including the highest content of icariin. Comparison among EKEs was performed in extraction methods (reflux, sonication, and percolation), composition of ethanol solvent (0, 20, 40, 60, 80, and 100%) and extraction temperature (80, 90, and 100℃). When extraction was performed with 100% ethanol at 100℃ under reflux method, the content of icariin was the highest. Also, the content of icariin in fractions were confirmed. Among the fractions, ethyl acetate fraction has the highest content of icariin. These results indicate EKE and icariin may have a synergistic action with PDE-5 inhibitor to improve erectile function and they also can be used as the basic data of natural product medicine.