A purified polysaccharide, designated as SE-WGP I , was isolated from wheat germ modified by steam explosion. The primary structure characteristics were determined by HPGPC, GC, periodate oxidation-Smith degradation, methylation analysis, FT-IR, NMR and Congo red test. The results showed that SE-WGP I was a homogeneous hetero-polysaccharide with the average molecular weight of 5.6 × 10 3 Da. The monosaccharide composition mainly consisted of glucose, arabinose and xylose with a molar ratio of 59.51: 20.71: 19.77. The main backbone of SE-WGP I consisted of →4,6)-α-D-Glc p (1→6)-α-D-Glc p (1→3)-β-D-Xyl p (1→5)-α-L-Ara f (1→ and the side chain was α-D-Glc p (1→ linked at the C4-position of →4,6)-α-D-Glc p (1→. SE-WGP I likely has a complex netted structure with triple helix conformation and good thermal stability. In addition, SE-WGP I had valid in vitro radical scavenging activities on DPPH and hydroxyl radicals. This study may provide structural information of SE-WGP I for its promising application in the fields of functional foods or medicines.