Some features of native enzyme's active site were used to conjunction with a chemical reagent or modifying group, which would generate new functionality different from the natural enzyme. In order to obtain an efficient catalyst, we have designed four different molecular size N-derivatives of modifiers and introduced them into the active site of papain to obtain new semisynthetic enzymes, which were used as catalyst in reduction of benzaldehyde to yield benzyl alcohol respectively, and the reactions carried out with recycling agent in 0.1 M phosphate buffer pH 6.5 at 37 A degrees C. The results had shown that a longer N-derivative of semisynthetic enzyme had higher catalytic activity. Furthermore, we propose a plausible model for the catalytic mechanism in the semisynthetic enzymes system.