The phase stability of intermetallic compounds of Co3X has been discussed with a focus on the γ' phase of Co3Al, Co3W and Co3(Al, W) with the L12 structure. The critical temperatures of the γ' phase of Co3Al, Co3W and Co3(Al, W) compounds are estimated to be about 870 , 980 and 1076 , respectively. The effect of alloying elements on the Co-Al-W -base alloys was found to be very similar to that of Ni-base superalloys, where Ti, Ta, Nb and V are the γ' stabilizing elements, while Mn, Fe and Cr are the γ' forming elements. The mechanical properties of Co-Al-W-base alloys were also found to be similar to those of Ni-base alloys. In particular, the flow strengths of Co-Al-W-base alloys at temperatures above 800 were comparable or higher than those of Ni-base superalloys, which implies that the Co-base superalloys strengthened by the γ' phase have great potential as a new type of high-temperature alloys.