Phase equilibria and martensitic and magnetic phase transformations of the β phase in the Co−Ni−Ga system have been investigated. It has been shown that the β phase is in equilibrium with the α-phase over a wide range compositions at 600–1100°C. The β phase exhibits both a paramagnetic-ferromagnetic transition and a thermoelastic martensitic transition from B2 to L10 structure. The Curie temperature TC increases with decreases with decreasing Ni content and with increasing Ga content. The composition region of the β phase exhibiting the thermoelatstic martensitic transformation from ferromagnetic austenite is located near the α+β two-phase region. TC and Ms of α+β two-phase alloys increase with increasing annealing temperature.