Thermodynamic calculation, SEM (scanning electron microscopy), TEM (transmission electron microscopy),XRD (X-ray diffraction), phase extraction, and chemical analysis were employed to study the phase stability and phase precipitation in a new Ni-Cr-Co base superalloy heat-treated at 704 and 760℃ for a long time. The results show that the precipitates of this new alloy heat-treated at standard annealing condition and heat-treated at 704 and 760℃ for a time up to 2000 h are γ′, MC, M23C6, and M6C, and η phase forms at grain boundaries and in matrix of samples heat-treated at 760℃as well. The mass fractions ofγ′ (+η), MC, M23C6, and M6C in all samples have no large changes with an increase in aging time, but γ′ precipitates grow obviously. The γ′-to-η transformation in the samples heat-treated at 760℃ took place with increasing aging time. The η precipitates form a Widmanstatten pattem and the γ′ phases have remelted partly in the samples heat-treated at 760℃. The alloy maintains a better microstructural stability during prolonged aging at 704℃, but a worse microstructural stability during prolonged aging at 760℃.