The impurity (ceftazidime methyl) produced in the synthesis of ceftazidime were analyzed and speculated, and 2-mercaptobenzothiazolyl-(Z)-(2-aminothiazol-4-yl)-2-(tert-butoxycarbonyl)isopropoxyiminoacetate (TAEM), a raw material for the synthesis of ceftazidime, was analyzed. The anal. results indicated that an unknown impurity existed in TAEM, and the exptl. results from LC-MS and HPLC determination indicated that the existence of the impurity was related to a reaction between TAEM and methanol solvent. Based on the above deduction, a compound with the same mol. weight from LC-MS anal. and same retention time in HPLC anal. of the unknown impurity in TAEM was synthesized.