Water soluble seeds polysaccharide was extracted from Cassia hirsuta Linn. plant by acid hydrolysis with sulphuric acid and identification by column and paper chromatographic analysis to be composed of D-galactose and D-mannose in 1:4 molar ratio. Methylation studies of seeds polysaccharide were carried out by Hakomari’s and Purdie’s method and its Infrared spectroscopy indicated that the polysaccharide was a galactomannan. D-mannose residues linked with (1®4)-β type on main polymer chain while D-galactose and D-mannose residues with (1®6)-α type linkages at non-reducing end. Methylated galactomannan on acid hydrolysis with sulphuric acid gave certain medicinal chemicals like methyl sugars as: 2, 3, 4, 6- tetra-Omethyl- D-galactose; 2,3,6-tri-O-methyl-D-mannose and 2,3-di-O-methyl-D-mannose is 1:3:1 molar ratio. On the basis of above finding methylation results a tentative polysaccharide structure has been proposed for the water soluble Cassia hirsuta Linn. seeds.