Simple and sensitive Gas chromatography with mass spectrometry (GC/MS) method was developed, optimized and validated for the determination of Mutagenic impurity i.e. Dimethyl sulphate (DMS) in Tipiracil Hydrochloride drug substance. The lower level of detection was achieved on Capillary GC column (Rtx-624, Fused silica capillary column; 30m length; 0.32mm internal diameter, coated with 6% Cyanopropylphenyl and 94% dimethyl polysiloxane stationary phase of 1.8µm film thickness) with Electron Impact ionization (EI) in Selective Ion Monitoring (SIM) mode. The developed method was validated for specificity, linearity, accuracy and precision. The obtained detection and quantitation limits of DMS were 0.2μg/g and 0.6μg/g respectively. The method was found to be linear in the range between 0.6μg/g and 29.9μg/g with correlation coefficient 1.0000. The average recovery range obtained for this impurity was 97.1%. The detail experimental approach is explained in this research paper.