The paper presents the results of plastic deformation by the multiple isothermal forging on microstructure and martensitic transformation in polycrystalline Ni54.1Mn19.6Ga24.6Si1.7 alloy. The method of plastic deformation was applied to Heusler alloys for the first time. By recording the temperature dependence of thermal expansion in the field of the existence of the martensitic transformation for Heusler alloys, the characteristic temperatures for given phase transformation in the alloy in as-cast state and after deformation processing are determined. After plastic deformation by forging at 680°С the bimodal grain structure of "necklace" type is formed.