The structures, the martensitic transformations and the magnetic properties of ferromagnetic shape memory alloy Mn2NiGa with the loading and the unloading of the external compressive stress are investigated. The plastic deformation occurring during the stressing causes a high level of dislocation defects existing in the sample. The residual internal stress results in a significant increase of the reversed transformation temperature. The threshold value of compressive pressure for causing the completely martensitic transformation at room temperature is identified to be 1.0 GPa based on the experimental measurements of XRD and magnetic coercivity. The coercivity of the martensitic phase increases from 50 Oe to 350 Oe due to the application of the external stress up to 2.0 GPa. It is also observed that the reversed martensitic transformation occurrs, when the sample is treated by heating up to about 730 K. This is attributed to the elimination of the dislocation by the annealing effect. Such a high reverse martensitic transformation temperature allows the measurement of the Curie temperature of martensitic phase and the obtained value is 530 K.