Adenovirus or naked plasmid DNA (pDNA) has been used to deliver the therapeutic gene into corpus cavernosum. However, the potential risks of viral vector and inefficiency of naked pDNA have limited their clinical application. In this study, water-soluble lipopolymer (WSLP) was evaluated as a gene carrier to corpus cavernosum. The WSLP/pDNA complex was transfected to smooth muscle cells in vitro. WSLP had high transfection efficiency, which was comparable to poly(ethylenimine) (PEI). In addition, WSLP had much less cytotoxicity than PEI, suggesting that WSLP is a safer carrier than PEI. To evaluate the transfection efficiency to corpus cavernosum, the WSLP/pDNA complex was injected into the rat corpus cavernosum. As a result, the WSLP/pDNA complex showed higher transfection efficiency than naked pDNA. In addition, the gene expression was dependent upon the dose of the complex. The results suggest that WSLP may be useful for gene therapy of erectile dysfunction.