Inconel 617 alloy holds significant application potential in Generation IV nuclear energy systems. The effects of Mo content on grain boundary carbide precipitation and high-temperature tensile properties of Inconel 617 alloy were studied by mechanical testing and advanced techniques such as SEM, TEM and EBSD. Experimental results showed that there were granular M₂₃C₆ carbide with the size of 0.1~0.5μm at GBs when the Mo content ranged from 8wt.% to 9wt.%. However, with Mo content increasing from 9.3wt.% to 9.6wt.%, massive M₂₃C₆ and M₆C carbides could be predominantly observed at GBs As Mo content increased from 8wt.% to 9.6wt.%, the elongation increased initially and then decreased, and 8.5wt.% to 9.3wt.% Mo alloys achieved optimal strength-ductility balance. As fractographic analysis revealed, precipitation of granular M₂₃C₆ at GBs effectively strengthened grain boundaries, which resulted in that the high-temperature tensile fracture showed transgranular fracture features. When massive M₂₃C₆ and M₆C precipitated at GBs, the initiation of intergranular crack was promoted and the intergranular fracture features appealed in the high-temperature tensile fracture. The Mo content of Inconel 617 alloy for high-temperature components in Generation IV nuclear systems shouldn’t exceed 9.3wt.% and it was better controlled between 8.5 ~9wt.%.