Shape memory alloys having ferromagnetic properties have received attention as new solid-state actuator materials. The recent our investigation of CoNiGa alloys was shown that the Co-Ni-Ga β alloys are promising ferromagnetic shape memory alloy. In this study, phase equilibria between β (B2) and γ (A1) phases at temperatures ranging from 873 to 1373 K and aging effect on martensitic transformation in the Co-Ni-Ga system have been investigated. It is shown that the composition region showing a ferromagnetic shape memory effect almost coincides with the β+γ two-phase region and that the martensitc transformation disappears with precipitation of ω phase induced by aging at 573 K.