Abstract: High performance size exclusion chromatography ( HPSEC) method was established to determine the relative molecular mass of flaxseed gum.Subsequently, monosaccharide composition of flaxseed gum was analyzed by gas chromatography after derivatization with aldononitrile acetate.Combined SEC columns of TSKgel G5000 PWXLand TSKgel G3000 PWXL, under conditions that ultrapure water as mobile phase, flow rate of 1.0 m L/min, sample concentration of 2.0 m L/min, column temperature of 40 ℃ and equipped with an evaporative light scattering detector, the relative molecular mass of dextran standards and flaxseed gum samples were determined. Results demonstrated that there was a good linear relationship between elution time and log M ( R2= 0.9842) .Precision, repeatability, stability and accuracy of this method were all acceptable.Relative molecular mass of flaxseed gum from hot water extraction process, microwave assisted hot water extraction process and commercially available were 7793, 2420, 10392/6166/89183 ku respectively. Mn, Mw, Mz and D were also calculated.Monosaccharide composition analysis indicated that flaxseed gum that self-prepared were composed of rhamnose, arabinose, xylose, galactose and glucose, while commercially available flaxseed gum contained mannose and fucose in addition.Monosaccharide composition of flaxseed gum from different sources were quite different.