Abstract: In this paper HPLC is applied to study radiochemical purity and chemical purity of 131I-sodium o-iodohippurate, and the reason for the error, which appeared in ordinary paper chromatography with benzene-ice acetic acid-water system as solvent for 131I-sodium o-io dohippurate, is analyzed. The method has the advantages of rapidity, sensitivity and exactness compared with the paper chromatography. It is suitable for routine quality control.