A heteropolysaccharide possessing strong inhibitory activity against cholesterol esterase, namely PS-A, was isolated from water extracts of Cordyceps sinensis by activity-guided fractionation. The molecular weight of PS-A was estimated as 4.6×10(5) Da. Compositional analysis revealed the presence of D-glucose, D-galactose, and D-mannose at a molar ratio of 2:1:1. The (1)H and (13)C nuclear magnetic resonance spectra, correlation spectroscopy, nuclear Overhauser effect spectroscopy, and heteronuclear multiple bond correlations experiment established the structure of PS-A as follows: →3)α-D-glcp(1→3)β-D-glcp(1→3)β-D-glcp(1→ 2 ↑ 1) α-D-manp Results of the in vitro enzyme assay showed strong inhibition of cholesterol esterase by PS-A, with an IC(50) value of 12.7 μg/mL, suggesting the possibility of its usefulness as a potential agent for control of hypercholesterolemia.