Abstract: Abnormal fracture of GH4169 alloy bolts occurred under the service condition of thermal power plants. To find out the abnormal fracture causes, failure analysis was carried out on the fatigue fracture of a GH4169 alloy bolt for combined high and medium pressure steam valve of ultra-supercritical unit by chemical composition testing, metallographic structure analysis, mechanical property testing and fracture surface analysis. The results indicate that the chemical composition of the failed GH4169 bolt meets the requirements of the general material grade but doesn’t reach the premium level. The hardness, room-temperature tensile properties and room-temperature impact properties are in compliance with the technical specifications. The failure cause of the bolt is that fatigue cracks initiated at the mechanical friction marks on the screw surface, and under the combined effect of cyclic loading, high temperature, oxidation and thermal stress, the material experienced intergranular embrittlement. Additionally, the bolt was subjected to abnormal lateral loads during service due to the assembly position and the frequent startup and shutdown of the thermal power plant. The cumulative effect of these factors reduced the service life of the bolt.