In this paper, two kinds of Ni-base alloys (alloy A and alloy B), which are alternative materials for superheater and reheater tubes of ultra-supercritical power station applications under high parameters, are systematically investigated including the phenomenon of grain boundary widening, the kinetics of the grain boundary widening process and the evolution mechanisms of this phenomenon. The results show as follows, the main precipitates in the widening grain boundary region are M₂₃C₆ carbide and the γ" phase. And the evolution of grain boundary broadening of A, B alloys with aging time follows the JMAK equations. The formation process of the widening grain boundaries is consist of three stages: first, the M₂₃C₆ carbides near the grain boundaries dissolve back inducing the coarsening of the M₂₃C₆ carbides at the grain boundary and the grain boundary migration; secondly, new M₂₃C₆ carbides are precipitated in the grain boundary migration area, and the M₂₃C₆ carbides in the grain boundary area are arranged in multiple layers; Thirdly, the γ" phase will precipitate at the M₂₃C₆ /γ interface with the decreasing coarsening rate of carbides at grain boundary, and the γ" phase will get bigger by short-circuit diffusion, resulting in the grain boundary widening to a great extent.