A water soluble crude polysaccharide has been isolated from the fruiting body of cultivated Cordyceps militarvs(L.) Link. The GC analysis indicated that it contained the monosaccharides of Gal, Man, Glc and GlcA. Their molar ratio is 1.00∶1.58∶7.89∶0.19. The crude polysaccharide was fractionated with ethanol to get three fractions, which were CM-Ⅰ, CM-Ⅱ and CM-Ⅲ. CM-Ⅱ was further purified to get one homogeneous fraction named CMB. The molecular weight was estimated to be 60000 by means of gel filtration. CMB is a greyish white powder. The GC analysis indicated that it was composed of Gal, Man, Glc and the molar ratio is 1.00∶1.38∶5.10. Partial hydrolysis with an acid, periodate oxidation, Smith degradation, IR, GC and methylation analyses, GC/MS analyses were used for the structure analyses of CMB. The results reveal that CMB is of a lower branched structure. The main chain is composed of (1→6)-linked-Man, (1→6)-linked-Glc, (1→4)-linked-Glc and (1→4)-linked-Gal. Some of (1→4)-linked-Glc and all (1→4)-linked-Gal residues are substituted at 3-O and 6-O respectively. (1→4)-Linked-Glc residues without branch probably exist with other monosaccharides alternately. On an average, there is one branch every ten hexose residues. The side chain is composed of major (1→4)-linked-Glc and minor (1→6)-linked-Glc. The nonreduced end is composed of Gal and Glc. The main glycosidic link of CMB is α configuration.