Abstract: Polysaccharides of Dendrobium was isolated by DEAE- 52 cellulose column and four kinds of polysaccharide (DO-1, DO-2, DO-3, DO-4) were got, then the component DO-1 was purified by Sephadex G-200 column, polysaccharide DOP was got. Using the method of pre-column derivatization with 1-phenyl-3-methyl-5-pyrazolone (PMP) for HPLC analysis and the results showed that, polysaccharides DOP was composed of D- mannose, L ( +) - rhamnose and D- glucose, and the external standard informed that D-mannose∶L (+) -rhamnose∶D-glucose=1.936∶0.856∶0.691. The antioxidant properties of polysaccharide DOP were determined, and the results showed that the polysaccharide DOP had strong scavenging effects to DPPH radicals.