This invention discloses a novel stable crystal form of imatinib mesylate, designated by us as &alpha；2 Form, which is stable at room temperature and even at higher temperatures up to 120° C. and accelerated stress conditions and, freely soluble in water. This invention also discloses a pharmaceutical composition containing the novel stable &alpha；2 form of Imatinib mesylate and other usually employed excipients, useful in the treatment of Chronic Myelogenous Leukemia (CML). This new &alpha；2 Form of imatinib mesylate is prepared by slurrying Imatinib base in isopropanol at room temperature followed by addition of methane sulfonic acid and maintaining 50-60 ° C. followed by filtration. This invention also discloses another process for the preparation of the novel, stable &alpha；2 crystalline form of Imatinib Mesylate by the conversion of Imatinib mesylate &bgr；-polymorphic modification by suspending it in water and organic solvents, distilling off water azeotropically, cooling and filtering to obtain the &alpha；2 crystal form.
