For antitumor drugs with an intracellular action site in the nucleus, effective internalization of the drugs into cancer cells and accumulation in the nucleus should be the determinant step for high antitumor activity. We synthesized a novel chitosan derivative by grafting stearic acid onto chitosan. The derivative can form self-aggregated micelles with about 50 nm size in the aqueous medium, and then can load a poorly soluble antitumor drug (doxorubicin, DOX) with high entrapment efficiency and drug loading. DOX release from the micelles was retarded significantly as a result of the encapsulation of the micelles. DOX concentration in nuclei was increased significantly via the transport of the micelles. Consequently, cytotoxicity of DOX loaded micelles was improved sharply due to the accumulation of the drug in its intracellular action site. The present micelles are a promising carrier candidate for effective therapy of antitumor drugs with the action site in the nucleus.