Nonstoichiometric Ni52+XMn23-xGa25 polycrystalline samples were made by arc melt method. The phase transformation temperatures of these alloys were determined by DSC and ac-susceptibility measurements. The results show that the substitution of Mn atoms by Ni at constant Ga content results in higher transformation temperatures and lower Curie temperatures. X-ray diffraction was used to identify the crystal structure. Ni52Mn23Ga25 and Ni53Mn22Ga25 possess for structure whereas Ni54Mn21Ga25 and Ni55Mn20Ga25 possess tetragonal structure at room temperature. Shape memory effect was investigated on polycrystalline Ni54Mn21Ga25 alloy. The strain of 1300ppm was obtained under zero applied magnetic field and 2500ppm was obtained under 1.2T applied magnetic field. The mechanism of magnetic field induced strain during phase transformation was also discussed.