A series of heat treatments were preformed to study the evolution of microstructure in an austenitic nickel-base Alloy 690 using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the pre-existed chromium carbides of cold-rolled Alloy 690 were dissolved into the austenitic matrix after high temperature mill annealing (MA) at 1353K for 5min. Mill-annealed and thermally treated Alloy 690 had intergranular M23C6-type carbides. The morphology of intergranular M23C6 varied from continuous, semi-continuous to discrete with the temperature of thermal-treatment (TT) increasing from 873K to 1073K. The holding time at temperature of 988K had less influence on the intergranular carbide appearance than that of temperature. The minimum chromium concentration in the vicinity of intergranular carbides was 22.3 wt% in the sample thermally treated at 715°C for 2h. With the holding time of TT increasing from 2h to 27h, Cr concentration increased gradually and the chromium depletion became wider.