Cellulase was covalently modified with a water soluble polymer; maleic acid methylvinylether copolymer. The molecular weight of the cellulase modified with the polymer was increased. Actually the crude modified enzyme includes the modified cellulase fraction and the unmodified cellulase fraction. And the specific activity of the crude modified cellulase was nearly equal to that of the native cellulase. Optimum pH of the modified cellulase shifted slightly to alkaline region and thermal stability was increased. When the crude modified cellulase was used in the cotton yarns treatment, the rate of weight loss for the yarns was as fast as that of native cellulase. However, the breaking strength of the yarns treated with the modified cellulase was higher than that of the yarns treated with the native cellulase. The rate of weight loss with the modified cellulase fraction was very slower than that with the native cellulase. These results might suggest that the modified cellulase could not easily penetrated into the amorphous region because of its high molecular weight.