Phase equilibria between the and 0 phases at 900 C in the Co-(10-70)Ni-Al-W system were determined by electron probe microanalysis (EPMA) and X-ray diffractometry (XRD). It was found that the 0 phase with L1 2 structure continuously exists from the Co side to the Ni side in Co-Ni-Al-W system and that it widens to the low W region with increasing Ni content. The partition of Al into the 0 phase increased with Ni content, while the W changed from a 0 former to a former by increase of Ni content. Differential scanning calorimetry (DSC) measurements also revealed that the 0 solvus temperature increases with Ni content, while the solidus temperature is hardly affected by such content. The lattice parameter of the and 0 phases and the mismatch decreased with increasing Ni content, which caused the morphologic change of the 0 precipitates from cubes to spheres.