Phase diagram and magnetic properties of Co2 (Cr1–xMnx) Ga Heusler alloys were experimentally established. Single phase was obtainable in the entire composition range, 0 ≤ x ≤ 1.0. Order–disorder phase transition temperature from L21 to B2 phase (TtL21/B2) and the Curie temperature (TC) increased with increase of Mn composition, in other words, with increase of number of valence electrons (Zt). The behavior of TC is general trend in the Co-based Heuselr alloys that were systematically investigated previously, whereas the concentration dependence of TtL21/B2 doesn't follow the Bragg–Williams–Gorsky approximation in Co2 (Cr1–xMnx) Ga alloys. Spontaneous magnetization also increases with increase of Zt with following the generalized Slater–Pauling rule, suggesting that the half-metal-type electronic structure is comparatively kept in the system.