Objective:To observe the effect of chitosan on tumor. Methods: Experiments of antitumor in vitro and in vivo were adopted in tumor-bearing mice. Results: Experiment in vitro demonstrated that chitosan had some killing effects on S 180、EAC and H 22 tumor cells, but it did not have directly killing effect on human body liver cancer. Experiment demonstrated that rate of inhibitory tumor was 66.0% or so, and it markedly increased thymus and spleen weights of tumor-bearing mice (TBM), and markedly inhanced the transformation rate of lymphocytes, it had markedly protective effect on thymus and spleen weights and different antagonisfic effect on the decrease in WBC、 neutrophil cell and loss of body weight induced from 5-Fu. Conclusion: Chitosan had more intensive antitumous effect and immunologic function of tumor-bearing mice and antagonisfic effect on bone marrow and immunologic inhibition induced from 5-Fu.