Lambda-carrageenan is a kind of sulfated galactan isolated from some red algae and reported to have more biological activities than other types, such as K-type [Güven KC, özsoy Y, Ulutin ON. Anticoagulant, fibrinolytic and antiaggregant activity of carrageenans and alginic acid. Bot Mar 1991; 34: 429-432]. The molecular weight (MW) of lambda-carrageenan reaches up to 600-700 kDa, the high MW decreased their solubility and limited their bioactivities. At the same time, the production and applications of lambda-carrageenan are limited for its some characteristics, such as the high viscosity, and the experiments of antitumor and immunomodulation also indicated that MW of polysaccharides had notably effect on the activities [Zhou G, Sun YP, Xin H, Zhang Y, Li Z, Xu Z. In vivo antitumor and immunomodulation activities of different molecular weight lambda-carrageenans from Chondrus ocellatus. Pharmacol Res 2004; 50(1) 47-53, Zhou G, Xin H, Sheng W, Sun Y, Li Z, Xu Z. In vivo growth-inhibition of S180 tumor by mixture of 5-Fu and low molecular lambda-carrageenans from C. ocellatus. Pharmacol Res 2005; 51(2) 153-157]. In this research, lambda-carrageenan was extracted from C. ocellatus, an important economic alga in China. Then some small MW samples of 650, 240, 140, 15 and 9.3 kDa are obtained by the method of microwave degradation, respectively. The tumor inhibiting activities of the low MW lambda-carrageenan and mixture of it and 5-Fu on transplanted H-22 tumor mice were investigated. The weight of immune organ, proliferation ratio of lymphocyte and concentration of Tumor Necrosis Factor-alpha (TNF-alpha) and histopathology of tumor from transplanted H-22 tumor mice were also determined. The results indicated that the degraded lambda-carrageenan could add the antitumor activities of 5-Fu and improve the immunocompetence damaged by 5-Fu.