Candida cylindracea lipase was modified by carboxylic-acid-terminated polymers having different molecular weights, i.e., poly[(N-acetylimino)ethylene] (2a), poly[(N-propionylimino)ethylene] (2b) and poly-[(N-sobutyrylimino)ethylene] (2c). The effect of N-acyl substituent of 2a-c on the enzymatic activity of modified lipase (4) for hydrolysis of olive oil and p-nitrophenyl propionate in an aqueous solution and for esterification between n-amyl alcohol and caprylic acid in chloroform, benzene, toluene, and isooctane was examined by using 2a-c of a similar MW (5000–6000). It was found that the active site reactivity of lipase was not as influenced by the chemical modification of lipase in an aqueous medium, and that the rates of esterification were remarkably enhanced, especially in benzene and toluene, by modification with 2b or 2c compared with the native lipase. The effects of the molecular weight of 2a-c and the average number of 2a-c per molecule of lipase on the enzymatic activity were also investigated.