Effects of Nb/Ti ratios on the microstructural evolutions of cast Ni-based suerpalloys during long-term thermal exposure are investigated by OM, SEM and TEM. The results show that Nb/Ti ratios have no influence on the evolution of γ? morphology and size during long-term thermal exposure. However, with decrease of Nb/Ti ratios in alloys, the volume fraction of γ? phase increases. Both parameters Nb/Ti and (Nb+Ti)/(W+Mo) of primary MC have a good linear relationship with Nb/Ti ratios in alloys. With decrease of Nb/Ti ratios in alloys, both parameters for primary MC linearly decrease and sequentially thermal stability of primary MC is weakened. However, the results also show that Nb/Ti and (Nb+Ti)/(W+Mo) ratios of primary MC are not the principle factors determining the thermal stability of primary MC. The degeneration degree of primary MC can be calculated by the volume fraction of primary MC before and after degeneration, while the thermal stability of primary MC can be quantitatively characterized by degeneration degree of primary MC. Furthermore, with decreased Nb/Ti ratios in alloys, the grain boundaries coarsen more severely during long-term thermal exposure. Meanwhile, precipitation tendency of M₂₃C₆ carbide on grain boundaries increases and that of M₆C carbide on grain boundaries decreases. However, the precipitation and evolution of μ phase during long-term thermal exposure is not affected by Nb/Ti ratios obviously.