During the brazing process, the high brazing temperature, the chemical and physical reaction between diamond and alloy, and the residual stress would result in the thermal damage of diamond. The morphology and mechanism of thermal damage is studied when the brazed diamond grits is brazed using Ni-Cr alloy in this paper. The results show that the manifestations of the thermal damage mainly include surface graphitization, erosion pits, microcrack and breaking. The graphitization only occurs on the surface of diamond grits. And the microcrack of diamond grits appears near interface.