Hypertension is a major health problem affecting all populations around the world. Angiotensin I-converting enzyme (ACE-I), an important enzyme in renin angiotensin system (RAS) catalysis, yields a product that increases blood pressure in our bodies. Many mushrooms have been reported to have blood pressure lowering properties. However, such activities have not been investigated in Lentinus polychrous Lev., one of the most consumed mushrooms in Thailand. In this study, an ACE-I inhibitory activity of crude protein extract of L. polychrous was examined. The crude protein was separated by a dialysis membrane of 10 kDa MWCO and an anion exchange chromatography on Q-Sepharose XL column, consecutively. The fraction with MW