Methoxypolyethylene glycol (Mr 5000),activated by cyanuric chloride and nitrophenol carbonate respectively,was used to modify subtilisin Carlsberg. The enzyme after PEG-modification maintained its catalytic activity both in aqueous solution and in 0rganic solvents,and an enhancement in activity and stability in aqueous solution was achieved:the k_cat increased accompanied with a lower activation energy,while K_m was not changed;the stability of the enzyme against both temperature and pH was greatly enhanced，but the optimal reaction temperature was unchanged. The pH dependence of the modified and unmodified subtilisin has also been compared.