Abstract: The weldability of a Ni-Fe base superalloy, one of the candidate materials for high temperature components of 700℃ ultra-supercritical coal plant, was studied in this paper.Based on welding thermal simulator experiments, the variation and evolution of microstructure and mechanical properties were analyzed for this Ni-Fe base superalloy under welding thermal cycle treatment condition. The microstructure analysis showed thatγ phase and MC carbide in Ni-Fe base superalloy dissolved during high temperature welding thermal simulation process. The creep rupture time of welding thermal cycled sample was severely shorter than that of base solutioned sample at 700℃/400 MPa, and was similar to that of base solutioned sample at 700℃/350 MPa and 700℃/300 MPa, respectively. During creep test at 700℃, M₂₃C₆ gradually precipitated at the grain boundaries of HAZ, while γ precipitated in the HAZ matrix. The precipitated M₂₃C₆ and γ resulted in an improvement of 700℃ creep rupture property of HAZ. The precipitation of M₂₃C₆ at grain boundaries during creep test was mainly due to the supply of the carbon from MC which was dissolved partially during former welding thermal cycle.