One of the most important topics in biochemistry is to understand the nature of enzyme reactions at the molecular 1evel.In contrast to the rapid progress of experimental technique,however,the atomic details of how enzyme really work still remain elusive.Owing to the rapid progress of theoretical / computational methods,reliable and quantitative description of enzymatic catalyses is becoming an important issue in modern theoretical chemistry.In this talk,the author presents our recent theoretical studies of the reaction mechanism of serine proteases,one of the most extensively studied family of enzymes.