Abstract: The cause of cracking of GH738 super alloy turbine casing in an aeroengine was analyzed by morphology observation, component analysis, microstructure observation, performance testing and thermal simulation experiments, and the crack type and generating mechanism were discussed. The results show that the crack type in the failed casing was fatigue crack, and the crack source was located on the outer surface of the casing fixture fringe. The local temperature of the casing fixture fringe was 850-900 ℃, and exceeded the allowable service temperature (810 ℃) of GH738 alloy, which led to decrease of volume fraction of γ' phase in microstructure, and the mechanical properties and fatigue resistance of the alloy decreased, thereby promoted the premature fatigue cracking of the casing.