The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated.Specimens with V-type notch were tested at the peak temperatures of 900,1000,and 1100℃.The crack growth curves as a function of the number of cycles were plotted.With the increase of peak temperature,the crack initiation life was shortened dramatically.Through optical microscopy (OM) and scanning electron microscopy (SEM) observation,it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles.The primary cracks generally propagated along a preferential direction.Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.