The deformation microstructures of a fourth-generation single-crystal (SC) Ni-base superalloy TMS-138 after low-cycle fatigue (LCF) have been studied. For comparison, a typical third-generation (TMS-75), and a second-generation (CMSX-4) Ni-base SC superalloy were also examined. The TMS-138 exhibited an excellent LCF behavior under a condition of R ratio of 0 at temperatures of 1073K and 1173K. The addition of refractory elements resulted in a remarkable improvement of LCF properties due to the different microstructure developed in TMS-138 as compared to the reference superalloys. The formation of a large number of stacking faults (SFs) in the matrix at 1073K and the more completed rafts and smaller interfacial dislocation spacing at 1373K were observed and discussed in relation to the performance of TMS-138 under a specific condition of R = 0. This work suggests that a fourth-generation SC superalloy TMS-138 is a promising candidate for applications in aeroengines.