The present invention relates to a stable oral pharmaceutical dosage form comprising imatinib mesylate and one or more pharmaceutically acceptable excipients wherein the amount of imatinib calculated as free base is more than 80% by weight based on the total weight of the oral pharmaceutical dosage form, and wherein the oral dosage form does not show polymorphic conversion after storage at 40° C. and 75% relative humidity for three months. It also relates to processes for the preparation thereof.