Abstract: The in-phase thermo-mechanical fatigue (IP TMF) behaviors in a temperature range of 600-900 ℃ for a nickel-base single crystal superalloy were studied by using scanning electron microscope (SEM), transmission electron microscope (TEM) and IP TMF test. The results show that the mean stress was compressive stress in the test. With the mechanical strain amplitude increasing, the fatigue life decreased, but the stress range and the plastic strain increased gradually. In the high temperature half cycle, this alloy displayed a cyclic softening. In contrast, the alloy showed a cyclic hardening in the low temperature half cycle. The main mechanism for IP TMF of the alloy was glide and cross-glide of a/2〈11-0〉111 dislocation in γ phase. The cracks initiated from micro-pores and propagated until the alloy fractured. The tensile stress played a leading role in the fracture of alloy.