Lambda-carrageenan is a sulfated galactan isolated from some red algae and have been reported to have many kinds of biological activities. Chondrus ocellatus is an important economic alga in China and many other parts of the world. The sample was obtained after lambda-carrageenan from the alga was degraded by microwave and its molecular weight is 9.3 kDa. In this study, tumor inhibiting activity of low molecular weight lambda-carrageenan and its mixture with 5-Fu on mice transplanted with S180 tumor was investigated. Weight of immune organ, proliferation ratio of lymphocyte concentration of TNF-alpha and histopathology of spleen and tumor from the transplanted S180 tumor mice were also determined. Results indicated that the degraded lambda-carrageenan could enhance antitumor activity of 5-Fu and improve immunocompetence damaged by 5-Fu.