An accurate, simple, rapid, precise and economical RP- HPLC method has been developed for the rapid estimation of ImatinibMesylate in bulk and pharmaceutical formulation. The separation was achieved on Phenomenex C18 G column ( 250 x 4.6 mm i.d, 5 μm), using Methanol : 1-octanesulphonic acid (0.05M) at PH-8 with KOH  70:30 (v/v) as mobile phase, at a flow rate of 1.0 ml/min. Detection was carried out at 269 nm and drug eluted with a retention time of 5.548 min. Beer’s law was obeyed in the concentration range of 2-12μg/ml with correlation coefficient 0.999. The method had been validated according to ICH guide lines for specificity, linearity, accuracy, precision, robustness, ruggedness, LOD and LOQ. The method was found to be specific, accurate, and precise, robust, rugged and sensitive. The proposed method was convenient for quantitative routine analysis and quality control of ImatinibMesylate in bulk and pharmaceutical dosage form. Key words: Imatinib Mesylate , RP-HPLC, Validation, 1-Octanesulphonic acid.