Objective: To investigate the inhibitory effects of chitosan oligosaccharide on tumor growth in vivo. Methods: Murine tumor cell line H22 was inoculated into. Then different doses of chitosan oligosaccharide were injected into the mice-bearing H22 liver carcinoma, IL-2 and IFN-? in the sera were measured by ELISA assay. The in vitro anti-proliferation activity of chitosan oligosaccharide on the lung carcinoma LH-7 cells was evaluated by MTT assay. Results: Chitosan oligosaccharide inhibited in vivo the growth of H22 tumor cells,with increasing the content of IL-2 and IFN-? in sera of the tumor-bearing mice. The weight of spleen and thymus of the mice were increased when compared with those of control group; tissue necrosis was observed in the tumor in situ; chitosan oligosaccharide also inhibited the growth of LH-7 cells in vitro. The inhibitory effect was shown in a concentration dependent pattern, but not correrlated with incubation time. The early characteristics of apoptosis of LH-7 cell could be observed under the transmission electronic microscopy. Conclusion: Chitosan oligosaccharide inhibits proliferation of tumor and improves immunity function of the hosts, and might induce apoptosis of LH-7 cells.