Alpha-form of imatinib mesylate and methods for its preparation

A process for preparation of crystalline imatinib mesylate in substantially pure α-form, the method comprising: Crystallizing of imatinib mesylate from a solution comprising an organic solvent, and dissolved therein imatinib methanesulfonic acid and seed crystals of substantially pure α-form of imatinib mesylate, wherein the seed crystals are added before imatinib mesylate begins to precipitate from the solution.