Abstract: Objective:To analyse the physicochemical and antioxidant characteristics of SDF from Ganoderma atrum. Methods:Water soluble dietary fiber(SDF) from Ganoderma atrum was extracted by hot water using the pilot scale experiment. Physico-chemical properties were characterized by using the infrared spectrum(IR),gas chromatography(GC) and so on. The scavenging rates of SDF derived from Ganoderma atrum against DPPH and OH· radicals and its ferric reducing/antioxidant power were investigated to evaluated the in vitro antioxidant activities of SDF. Results:The total sugar,protein and uronic acid contents of SDF were 47.3%,7.0%and 5.7%,successively. As shown by infrared spectrum(IR),SDF possessed characteristic absorptions of polysaccharide and β-glucosidic linkage. Mannose,glucose and galactose were the main monosaccharide of SDF with molar ratio of 1∶4.4∶3,according to gas chromatography(GC). The antioxidant activity analysis showed that the SDF possessed good antioxidant activities with dose-dependence via DPPH· radicals,OH· radicals and ferric reducing/antioxidant power models. Conclusion:The SDF extracted from Ganoderam atrum possessed characteristic absorptions of polysaccharide. Mannose,glucose and galactose were the mainly monosaccharides.SDF showed a certain extent antioxidant effect in vitro. It had the value of development and utilization.