Abstract: This study was conducted to investigate the monosaccharide composition and antioxidant activity of the polysaccharide from Antrodia cinnamomea(ACP). ACP was obtained with water extraction and ethanol precipitation method. The total sugar content of ACP was determined by using glucose as the standard substance and phenol-sulfuric acid method. ACP was hydrolyzed with trifluoroacetic acid,which monosaccharide compositions were analyzed by glyc-nitrile derivatization and gas chromatography-mass spectrometer(GC-MS)method. The antioxidant capacity of ACP in vitro was evaluated by DPPH,ABTS and OH free radicals scavenging assay. Results showed that total sugar contents of ACP was 55.42%±2.13% with the yield of 3.02%,mainly composed of galactose,glucose,mannose with an average molar ratio of 1:7.14:0.96,respectively. The result of antioxidant tests in vitro showed that ACP scavenged free radicals viz. DPPH free radical,ABTS free radical and OH free radical with IC₅₀ values of(0.584±0.008),(1.182±0.058),(1.384±0.133) mg/mL,respectively. GC-MS method employed for analysis of the monosaccharide compositions of ACP was simple,rapid and sensitive. ACP exhibited a certain degree of scavenging activity for the three free radicals.