Reactive aqueous two-phase partition of cellulase by modification with amphiphilic copolymer was studied. The modifier of cellulase could be directly used as a polymer of two-phase system. The maleic acid anhydride functional group of the synthetic copolymer covalently coupled with the amino groups of cellulase molecule. The degree of modification of the amino groups was controlled by the types of synthetic copolymer of the weight ratio of a synthetic copolymer to cellulase. As the degree of modification of cellulase increased, the modified cellulase moved to the copolymer phase of the copolymer/dextran two-phase system.