A water-soluble polysaccharide (MOWP) and an alkali-soluble polysaccharide (MOAP) were isolated from Mallotus oblongifolius. The MOWP and MOAP yields were 6.00% and 3.07%, respectively. The molecular weights of the MOWP and MOAP were determined by a high performance gel permeation chromatography assay. The molecular weights of the MOWPs were 906 kDa and 49 kDa, and the molecular weight of the MOAP was 95 kDa. The monosaccharide composition was determined by a pre-column derivatization assay. Based on the monosaccharide composition analysis, the MOWP was mainly comprised of glucose, galactose, and xylose. MOAP was mainly comprised of mannose, galactose, and xylose. The glycosidic bonds of MOWPs and MOAPs were determined by a methylation assay. The branched MOWPs and MOAPs were composed mainly of →3)-Xylf-(1→, →3)-Galp-(1→ and →3)-Glcp-(1→ in different ratios. MOWPs and MOAPs exhibited moderate antioxidant capacities. MOWPs showed better DPPH scavenging activity than did MOAPs. MOAPs showed better reducing power and oxygen radical absorbance capacity than did MOWPs.