Abstract: The ratchet test and fatigue test were carried out on 18CrNiMo7-6 high-speed train gear steel at different maximum cyclic stresses (600-880 MPa) and stress ratio of 0.1. The fatigue limit was predicted by fitting the ratchet strain difference and temperature rise to the maximum cyclic stress at the stable stage, and then was predicted with the fracture fatigue entropy calculated from ratchet strain difference and temperature rise. The predicted results by different methods were compared with test results. The results show that according to the linear fitting results between the ratchet strain difference and the temperature rise and the maximum cyclic stress, the fatigue limits were 664.9, 681.4 MPa, respectively, and the relative errors of predicted fatigue limit with those obtained by fatigue tests were 3.50% and 1.11%, respectively, indicating that these two methods could predict the fatigue limit with high accuracy. When the maximum cyclic stress was 673.2 MPa, the fracture fatigue entropy changed from less than 0.1 MJ·m^-3·K^-1 to 0.46 MJ·m^-3·K^-1 suddenly. The fatigue limit predicted by this method was 673.2 MPa, and the relative error of the predicted fatigue limit with the fatigue test result was 2.3%, indicating high prediction accuracy.