The intrinsic relationship between the microstructure evolution and thermal fatigue behavior of a single-crystal cobalt-base superalloy has been investigated.The thermal fatigue tests are performed cyclically between room temperature and 1050 ℃ using V-notch plate specimens.Three states of thermal fatigue specimens are selected:the as-cast,solutionized as well as aged states.The solution treatment is carried out at 1260 ℃ for 24 h,which results in the dissolution of most of interdendritic continuous primary carbides.The subsequent aging treatment is carried out at 1100 ℃ for 100 h after solution treatment,resulting in the precipitation of a profusion of chain-and point-like M23C6 carbides in the matrix.The results indicate that the heat treatment can improve the thermal fatigue properties of the alloy and the effect of the solution treatment is more prominent than that of the aging treatment.The coarse and continuously distributed primary carbides in the as-cast state are changed into small and discontinuous distribution by heat treatment,which is the dominant factor in the improvement of thermal fatigue property.Additionally,the effect of oxidation behavior during thermal fatigue test on the thermal fatigue behavior is also studied.