Cordyceps militaris, an entomophathogenic fungus belonging to the class Ascomycetes, has been reported to have beneficial biological activities such as hypoglycemic, anti-inflammatory, anti-tumor, anti-metastatic, hypolipidemic, immunomodulatory, and antioxidant effect. In this study, the crude water-soluble polysaccharides CMP, which was obtained from the fruiting body of C. militaris by hot water extraction and ethanol precipitation, was fractionated by DEAE cellulose and Sepharose CL-6B column chromatography. This process resulted in three polysaccharide fractions, termed CMP Fr I, CMP Fr II, and CMP Fr III. Of these fractions, CMP Fr II, with an average molecular weight of 127 kDa, was able to upregulate effectively the phenotypic functions of macrophages such as NO production and cytokine expression. The chemical property of the stimulatory polysaccharide, CMP Fr II, was determined based on monosaccharide composition, which consisted of glucose (56.4 %), galactose (26.4 %), and mannose (17.2%). Its structural characteristics were investigated by a combination of chemical and instrumental analyses, including methylation, reductive cleavage, acetylation, Fourier transform infrared spectroscopy (FT-IR), and gas chromatography-mass spectrometry (GC-MS). Results indicated that CMP Fr II consisted of the (1-->4) or (1-->2) linked glucopyranosyl or galactopyranosyl residue with a (1-->2) or (1-->6) linked mannopyranosyl, glucopyranosyl or galactopyranosyl residue as a side chain. The configuration of the beta-linkage and random coil conformation of CMP Fr II were confirmed using a Fungi Fluor kit and Congo Red reagent, respectively.