Phase equilibria of Ni–Mn–Ga ternary system at 800 °C, 900 °C and 1000 °C were experimentally investigated by the diffusion couples and equilibrated alloys. The results show that the bcc phase possesses a wide composition range at high temperature. And the liquid and bcc phases extend to the Ni–Mn side and the region of bcc phase becomes longer and narrower with increasing temperature. A new ternary intermetallic compound (Ni, Mn) 2 Ga with the Mn 2 Ge-type hexagonal structure is present at 800 °C, but it disappears at 900 °C and 1000 °C. Based on the obtained information of phase equilibria, the composition dependence of martensitic transformation on the composition region of Ni 2 MnGa alloys was discussed. The present study may be useful for structural analysis and phase identification, as well as estimation of martensitic transformation temperatures of Ni–Mn–Ga alloys.