The derivatives of polyethylene glycol (PEG) were prepared by reacting PEG with propylene oxide to enhance its hydrophobicity and introduce a branched structure. The PEG derivatives were activated with cyanuric chloride and used to modify the lipase from Candida rugosa. The maximum specific activity of lipase modified with the PEG derivatives was about 2-fold of that modified with PEG for the esterification of oleic acid and lauryl alcohol in hexane.