Low pressure turbine guide serves in extreme conditions. Repeated heating/cooling cycles have a significant impact on microstructure and properties of alloy used in turbine guide vane. In this paper, the microstructure, mechanical properties and thermal fatigue behaviors of the serviced K417G guide vane were systematically investigated. The results show that coarsening of γ′ phase for the serviced K417G alloy has significantly degraded the microstructure. The strengthening effect of γ′ phase in the serviced K417G alloy has significantly weakened. Due to low bonding strength of precipitate interfaces and grain boundaries at high temperature, the tensile strength and yield strength of the K417G guide vane have decreased with the increase of temperature. The thermal fatigue crack forming process of K417G alloy under thermal stress has been observed with time. With increase of thermal cycles, the thermal barrier on surface of the vane has fallen off due to thermal fatigue stress, which leads to oxidation of surface of the alloy. The thermal fatigue crack sources can be initiated in the place where the oxide particles fractured and fell off. The a~N and da/dN~N curves obtained from thermal fatigue tests indicate that the crack growth rate decreases with the increase of thermal cycles, which is resulted from the release of thermal stress due to presence of secondary cracks.