A method is provided for preparing a polymer derivative of docetaxel provided by an ester linkage between at least one hydroxyl group of docetaxel and a carboxyl group of an aspartic acid side chain of a block copolymer containing polyethylene glycol and polyaspartic acid, which method includes a step of adjusting the ratio and/or the number of docetaxel molecules linked to the block copolymer to thereby control the release rate of docetaxel from the resultant polymer derivative of docetaxel.
