The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni–Mo–Cr superalloys with different silicon contents(0 and 0.46 wt%) was investigated. Experimental results showed granular M2C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M2C carbides will transform into plate-like M6C carbides as exposure time increases. For the Si-containing alloys,only the granular M6C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure. The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni-Mo-Cr superalloys with different silicon contents (0 and 0.46 wt%) was investigated. Grain boundaries after exposure for 100 h for the non-silicon alloy. For the Si-containing alloys, only the granular M6C carbides formed at the grain The coarsening of the grain boundary carbides occurred in both alloys with the increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloys. tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more s eriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.