The hydrophilic copolymer, polyethylene oxide (PEO) allyl ester-maleic anhydride (MA), copolymer was used to modify the lipase from Candida rugosa. MA, in a functional group, reacts easily with amino acids of lipase. The degree of modification (DM) was varied by changing the weight ratio of copolymer to protein of lipase over the range of 10-120 (w/w). The specific activity decreased as DM increased. At the maximum modification degree of 35%, the modified lipase retained more than 65% of the unmodified native lipase activity. The modified lipase displayed a high stability of activity against temperature and pH. The remaining activity of modified lipase was about 2-4 fold of that of native lipase in the severe pH and temperature condition. Finally, it showed 20% greater reaction of substrate at 10 hr than in the case where native lipase was used.