In this work, crude polysaccharide extracts were obtained from mycelia of the edible fungus Ganoderma capense (Lloyd) Teng. After removal of proteins by the Sevage method, fractionation and purification by anion-exchange and gel-permeation chromatography, a polysaccharide (GCPB-3) was isolated. The relative molecular weight of GCPB-3 was 124 kDa determined by high performance gel permeation chromatography (HPGPC). The homogeneous polysaccharide was composed of d-xylose and l-arabinose in the ratio of 1:1, and showed a specific optical rotation of [α]D25 = +145°(c 1.0, HO). Its structural features were determined by monosaccharide analysis, partial acid hydrolysis, methylation analysis, periodic acid oxidation, gas chromatography-mass spectrometry (GC-MS), Fourier transform-infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy (H, C, HMQC and HMBC). The results characterized GCPB-3 as a heteropolysaccharide with backbone consisting of β-l-Arap and β-d-Xylp, linked with 1→4 sugar bonds. Interestingly, GCPB-3 showed some DPPH•- and hydroxy-radical scavenging activities.