Abstract: A method of high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) was proposed to determine ethylenediamine, a genotoxic impurity in sitagliptin phosphate, by derivatization of ethylenediamine with p-toluenesulfonyl chloride. Waters CORTECS^® C₁₈ column was used as stationary phase, and the mixture of 5 mmol·L^-1 ammonium acetate-acetic acid buffer solution (pH 5.6) and acetonitrile at different volume ratios was used as mobile phase for gradient elution. Electric spray ion source in positive ion mode (ESI⁺) and select reaction monitoring (SRM) were adopted in MS/MS analysis. Linear ranges of ethylenediamine was found in the range of 0.50-33.5 μg·L^-1, with detection limit (3.3s/k) of 0.55 μg·L^-1. Test for recovery was made by standard addition method on the matrix of blank sample, giving the results in the range of 102%-103%. Repeatability and reproducibility tests were carried out on the 6 spiked test solution prepared in parallel, and RSDs of the determined values were less than 3.0%.