Abstract: In order to understand the fundamental causes of service damage in isothermal forging molds, for the isothermal forging molds with surface cracking after service, the damage forms and causes were studied by macroscopic corrosion, scanning electron microscopy, energy spectrum analysis and mechanical property testing methods. The results show that there are no casting defects such as porosities or cold insulation on the surface and inside of the mold, and wear or deformation was not observed after service. The damage to the mold occurs on the upper surface, mainly manifests as cracks caused by the high-temperature oxidation and the cold and hot fatigue during service, with the depth of 10-30 mm. The high-temperature tensile properties and durability properties for the remaining parts of the mold are higher than that before service, and the reason is that the mold alloy further precipitates nanoscale secondary γ′ phase and granular M6C during service, which plays a strengthening role. In order to improve the service life of the mold, rapid cooling and heating should be avoided during service, and the casting process should be controlled to avoid the generation of initial large block shaped M6C.