Abstract: PSP (Polysaccharide of Spirulina Platensis),a water-soluble polysaccharide,wasisolated and purified from the water extract of Spirulina platensis by chromatography on DEAE-cellulosecolumn and sephadex G-75 column.The structure of the polysaccharide of Spirulina platensis (PSP) has been investigated by using thetechniques of periedate oxidation, Smith degradation ,acetolysis,IR and 1HNMR data,The results ofthese studies indicate that it is a branched heteropolysaccharide.Its monosaccharide composition was identified by paper chromatography,thin-layer chromatography and HPLC. The results indicate thatPsP consists of L-rhamnose,D-xylose,D-glucose and an unknown sugar.