Propagation behaviours of naturally initiated small crack in a polycrystalline Ni-base superalloy,IN738LC,under thermo-mechanical fatigue loading were investigated.TMF and isothermal low-cycle fatigue (LCF) tests were carried out under a strain ratio of-1 at temperature ranged between 400 and 880 ℃.In the former the tests were performed under the in-phase (IP) and out-of-phase (OP) conditions in which the phase difference between strain and temperature were 0°and 180°,respectively.The experimental results revealed that the initiation and propagation morphologies of the naturally initiated crack were affected by thermo-mechanical loading condition.The small cracks initiated and propagated at the grain boundaries perpendicular to the loading axis under the LCF and IP conditions.On the other hand,they initiated and propagated with the transgranular modes under the OP condition.The rates of small cracks under IP-TMF and OP-TMF loading are almost comparable to those under LCF loading,i.e.,the small crack propagation rates can be expressed with a unique curve under LCF,IP-TMF,and OP-TMF conditions,when the propagation rates are correlated with the fatigue J integral range.The crack initiation and propagation mechanisms were discussed based on the crystallographic investigations by means of the EBSD analysis.