ASW0 is a polysaccharide derived from the perennial herb Aster scaber Thunberg. We isolated ASW0, a fraction of crude polysaccharide, by means of ethanol precipitation and dialysis after hot water extraction to investigate its physicochemical properties and immunostimulatory effects. ASW0 contains neutral sugar (45.7%), acidic sugar (51.6%), protein (2.3%), and 2-keto-3-deoxy-D-manno-octonate (KDO) (0.4%). The neutral sugar in ASW0 (in mole percentage) was mainly composed of arabinose (34.5 mol%), glucose (31.1 mol%), galactose (14.9 mol%), and rhamnose (8.1 mol%), which are characteristic of pectic polysaccharides. ASW0 also contained small amounts of xylose, mannose, and fucose. The anti-complementary activity of ASW-0 was similar to that of polysaccharide K (used as positive control). ASW0 exhibited no cytotoxicity in RAW 264.7 macrophages and dramatically increased nitric oxide (NO) production in a dose dependent manner (0.3~30 μg/ mL). Also, macrophages stimulated with ASW0 showed enhanced production of immunostimulatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) in a dose dependent manner. These results suggest that the ASW0 have a potent immunostimulatory effect and can be used as a natural immune health ingredient