Abstract: The microstructure evolution in a new Ni-Fe-base superalloy HT700T during creep deformation at 750 ℃/120 MPa is investigated by a transmission electron microscope. Experimental results show that dislocation climb controls the creep deformation, and anti-phase boundary shearing, stacking fault shearing as well as dislocation looping also take place in the regions near the carbides and grain boundaries, although the applied stress is much lower than the yield strength of the alloy at this temperature. Meanwhile, it is also found that as creep deformation proceeds, the γ′ precipitates grow up, and more and more dislocation loops are visible in the alloy. Based on the experimental observations, the relationship among the microstructure and the deformation mechanisms as well as the creep deformation behavior is discussed.