Objective: A performance liquid chromatog.-mass spectrometry (LC-MS) method was developed to determine the potential mutagenic impurity (Et 4-(2-phthalimidoethoxy)acetylacetate) residues in amlodipine besylate.Methods: The samples were dissolved in 90% acetonitrile and prepared into solution, and the solutions were then separated on Agilent Zorbax SB-C18 (4.6 mm×250 mm, 5μm) reversed-phase chromatog. column using a binary solvent composed of 0.1% formic acid in water and acetonitrile as mobile phase by gradient elution.Multiple reaction monitoring was used to acquire mass spectrometric data.External standard method was adopted for quantification.Results: The good linearity was got in the range of 3.1∼229 ng/mL, correlation coefficients was more than 0.990.Limit of detection was 0.0092 ng, and limit of quantitation was 0.0305 ng.The average recoveries ranged from 96.1%∼106.6% for four spiked levels in samples, and the relative standard deviation (RSD) was less than 5%.Conclusions: The developed method was simple, efficient, selective and sensitive, which can be used for accurate quant. measurement of the potential mutagenic impurity (Et 4-(2-phthalimidoethoxy)acetylacetate) residues in amlodipine besylate.