Purpose　The aim is to study the activity and the sildenafil selective inhibition of PDE5. Methods　The PDE isoenzymes were purified from bovine penis corpus cavernosum tissue by FPLC system. PDE activity was assayed by using 3　H-cGMP as substrate, the PDE isoenzymes hydrolyzed it to 3　H-GMP, and 3　H-GMP was further hydrolyzed to 3　H-guanosine by 5′-nuclease of snake venom. Add scintillation cocktail to observe the PDE isoenzymes activity. The selective inhibitor sildenafil of different concentrations were used to observe the inhibition of PDE5. Data replotted according to procedure of Dixon plots.Results　Three PDE isoenzyme peaks were purificated from bovine corpus cavernosum. The PDE of the third peak had the strongest activity of cGMP hydrolyzation which could be inhibited by sildenafil apparently.Conclusion　Since PDE5 was mainly found in corpus cavernosum tissue of mammalian, and sildenafil was a selective inhibitor of PDE5. It was suggested that the third peak was PDE5. The result was in agreement with the article reported.