A new high sensitive and specific LC-MS/MS (Liquid chromatography coupled with tandem mass spectrometric detection) method was developed and validated for the determination of 2-Chloro-N,N’-diethylacetamide (CDEA), a genotoxic impurity, in Entacapone drug substance. Hitherto there is a method known for identification of DECA in Entacapone. The successful separation of Entacapone and 2-Chloro-N,N’-diethylacetamide (CDEA) was achieved using Zorbax SB Aq column (Size: 4.6 x 250 mm, 5 µm particle size) with mobile phase consisting of 0.1% formic acid in water (50:50 v/v) as Mobile phase-A and Acetonitrile (100% v/v) as Mobile phase-B. High sensitive detection was achieved with “Applied Biosystems, Sciex, API-4000” Mass spectrometer and “API 4000, MDS Sciex, Toronto/Canada” Mass Detectors. As part of the method validation, system suitability, specificity, limit of detection (LOD), limit of quantification (LOQ), linearity, accuracy, precision, and stability of stock solutions were determined.