Abstract: An LC-MS/MS method was established for the identification of the related substances in cefmetazole sodium.HPLC separation was carried out on an ODS column（250 mm×4.6 mm，5 μm）with a gradient elution with a mobile phase consisting of 1% formic acid aqueous solution and pure acetonitrile.80 percent of the eluent was monitored with PDA,and the rest was detected by tandem mass spectrometry.The PDA,parent ions and the corresponding product ions spectra of all the related substances in cefmetazole sodium were determined and elucidated.Good resolution of cefmetazole sodium and its main related substances was achieved.Eight related substances in cefmetazole sodium were detected by LC-MS/MS.The established method is effective in the separation and identification of the related substances in cefmetazole sodium and the results are useful for its quality control.