The influences of Ni on the phase transformation temperatures, γ /γ′ two-phase microstructural evolution, γ′ dissolution behavior and microhardness have been investigated in four Co-Al-W base alloys containing various Ni contents (15%~45%, atomic fraction). The results show that the γ′ solvus temperatures continuously increase and the solidus temperatures are nearly unchanged with increasing the Ni content. The γ/γ′ two-phase microstructure is generated in four experimental alloys after the heat treatment at 900 ℃ for 50 h, whereas the γ′ morphology changes from cuboidal to nearly spherical and the γ′ volume fraction reduces as the Ni content increases. When prolonged heat treatment at 900 ℃ for 300 h is employed, no significant change in the γ′ morphology is observed in four experimental alloys but the γ′ volume fraction decreases to different degree as a function of Ni concentration. High temperature treatments at 970~1060 ℃ are conducted after experimental alloys are heat treated at 900 ℃ for 300 h. In the high temperature range, the dissolution of the γ′ phase is more pronounced as the temperature elevates, whilst the γ′ morphology becomes spherical and cuboidal in alloys containing the low and high levels of Ni, respectively. The microhardness results of the experimental alloys after heat treatment at 900 ℃ for 50 h and 300 h indicate that the microhardness is lowered in alloys with higher Ni content, but it increases as the heat treatment time is prolonged.