Abstract: An intermediate product used for the preparation of cephalosporin antibiotics,α-(2-aminothiazol-4-yl)-α-(tert.-butoxy-carbonyl)-iso-propoxyimino-acetic acid (ATIA) was determined by RP-HPLC.SpherigelTM C18 (250 mm×4.6 mm,5 μm) column was used as the stationary phase,and 0.05 g·L-1 tetrabutylammonium bromide solution disolved in CH3OH-H2O (6+4) was used as the mobile phase,with a flow-rate of 1 mL·min-1 and at column temperature of 30 ℃.UV-detection at 260 nm and the external standard method were used in the determination.Linear relationship between the peak area and mass of ATIA was found in the range of 0.001-1.0 mg corresponding to the regression equation A=31 475.6 m+1 786.7 (r=0.999 7).Precision was tested at 0.06 mg of ATIA for 6 determinations,and value of RSD obtained was 3.7%.Detection limit (3S) was found to be 0.6 μg according to the definition of IUPAC.Recovery was tested by standard addition method,and values found were in the range of 98.8%-104.0%.