Lipase was covalently immobilized on silicas with differently controlled pore size distribution and utilized in benzene to catalyze esterification. When silicas were converted to alkylamino derivatives in preparation for immobilization, their physical properties; such as average pore size, changed remarkably, but were relatively unchanged during the succeeding immobilization. Thus the derivatives determined immobilized quantity of the enzyme with their physical properties such that they generally immobilized more enzyme as their pores became larger in size and in volume. The activity of the immobilized enzyme depended on the immobilized quantity of the enzyme, water content, resistance of intra-mass transfer affected by pore size distribution, chemical properties of the silica surface and an over-crowding effect possibly emerging among immobilized lipase molecules. Among these factors, the first two were principal ones and activity was highest at the water content where their pores were filled, being generally higher with an increase in the immobilized quantity of the enzyme.