The structure, the martensitic transformation and the magnetic properties of ferromagnetic shape memory alloy Mn2NiGa aged at low temperature are investigated. The lattice distortion and the internal stress are generated in the samples due to the fine precipitation generated during the aging treatment. When the precipitation concentration exceeds the tolerance limit of the parent phase lattice, it causes the martensitic transformation temperature to increase obviously, and thus the parent phase transforms to martensitic phase with a large coercivity up to 900 Oe. Because of the existence of internal stress, the reverse martensitic transformation is shifted toward high temperature up to 485 K. By extrapolating the experimental data, the Curie temperature of martensite is estimated at 530 K. The internal stress disappears and the martensite turns to be of the parent phase due to the coarsening of the precipitation at higher temperature. Two coarsening threshold temperatures are found to be 423 K and 485 K which are the temperature sensitive and the aging time sensitive, respectively.