OBJECTIVE： To study the antitumor effect of phellinus linteus polysaccharide on sarcoma S180 cells in vivo and in vitro. METHODS： Sarcoma S180 cells in logarithmic growth period were selected， adding into 0 （blank control）， 2， 4， 8 mg/mL phellinus linteus polysaccharide solution and respectively culturing for 12， 24， 36， 48 h. The in vitro proliferation inhibition rate of cells was determined by MTT method； its apoptotic morphology was observed by fluorescence staining and cell apoptosis rate was detected by flow cytometry. S180 tumor-bearing mice models were established and randomly divided into control group， phellinus linteus polysaccharide high-dose， medium-dose， low-dose groups （400， 200， 100 mg/kg）， 10 in each group. Model mice were intragastrically administrated related medicined， once a day， for 12 d. Mice were executed after 24 h of last administration， tumor weight was determined， tumor inhibition rate was calculated. Immunohistochemistry was conducted to detect the tumor suppressor gene PTEN and oncogene C-myc protein expressions in tumor tissue. RESULTS： Compared with blank control group， phellinus linteus polysaccharide can increase the proliferation inhibition rate of S180 cells and induce the increase of apoptosis rate （P＜0.05 or P＜0.01）， showing a concentration-time manner. Compared with control group， the tumor inhibition rates in phellinus linteus polysaccharide groups were obviously increased （P＜0.01）， PTEN protein expressions were strengthened （P＜0.05 or P＜0.01） and C-myc protein expressions were weakened （P＜0.05）. CONCLUSIONS： Phellinus linteus polysaccharide shows antitumor effect in vivo and in vitro， which can up-regulate the PTEN， down-regulate C-myc protein expressions.