Ophiocordyceps sinensis (=Cordyceps sinensis) has been known for its various medicinal properties, in particular immunomodulatory activities associated with its polysaccharides. In this study, the fruiting body of O. sinensis cultivar OCS02® was investigated for its chemical composition and monosaccharide profile. Cold water extract (CWE) obtained from this fruiting body was fractionated by molecular weight (MW) into high (HMW), medium (MMW), and low (LMW) fractions. Polysaccharides in the extract and fractions were identified as heteroglycans containing mostly glucose and mannose with small amounts of galactose, fucose, arabinose, and xylose. The immunomodulatory potential of these heteroglycans was evaluated by induction of cytokine/chemokine secretion using murine macrophage RAW 264.7. All treatments showed significant modulation of IL-6, IL-9, MIP-2, and TIMP-1, especially for CWE, HMW, and MMW, which might be due to their high ratios of glucose and the presence of protein. Further investigation on the structure-function relationship of these fruiting body polysaccharide fractions is needed to delineate the underlying mechanism of their immunomodulatory effect both in vitro and in vivo.