Abstract: In a BR buffer solution of pH 4.10,an ion association complex was formed between heparin and thionine,resulting the color-fading of the basic dye,thionine.Decrease of the intensity of absorbance of thionine was found to be directly proportional to the concentration of heparin in the range of 0-60 μg/25 mL.The value of molar absorptivity was found to be 3.58×106L·mol-1·cm-1.Effects of surfactants and co-existing substances were studied,and the results obtained showed its acceptable selectivity.The proposed method has been applied to the determination of potency of heparin injection liquor,giving RSD′s less than 3%,and recoveries in the range of 96.5%-101.9%.