Abstract: A series of Fe-Mo-C steels with the Mo mass fraction of 0.1%to 0.8%were designed to study the high-temperature strengthening mechanisms of fire-resistant steels with Mo addition.Two heat treatments were carried out on the Fe-Mo-C steels to investigate the strength contributions of Mo addition from solid-solution strengthening and bainite strengthening.It is found that Mo can remarkably increase the high-temperature strength of the Fe-Mo-C steels.The dominant strengthening mechanism is solid-solution strengthening.The contribution of Mo addition to strengthening the Fe-Mo-C steels shows a rapid increase with the mass fraction of Mo no more than 0.5%,and the yield strength at 600℃ goes up by a significant 13.71 MPa per 0.1%Mo addition.But the high-temperature strength slowly increases when the mass fraction of Mo is more than 0.5%.Bainite strengthening also has great effect on the high-temperature strength of the Fe-Mo-C steels,and the strength increases remarkably when the volume fraction of bainite exceeds 20%.