Abstract: The high-temperature mechanical properties of 12 Mn steel round billet were tested by using a Gleeble-1500 D thermal/force simulation tester.Tensile strength,the reduction of area(ψ) and stress-strain curve in the range of 600~1 300℃ were obtained.The results show that of 12 Mn steel round billet has high strength.The tensile strength at 650℃ is 275 MPa,and it decreases with increasing temperature.The third brittleness temperature zone of the round billet is wide with temperature range at 600~750℃,and the reduction of area(ψ) in the temperature range is less than 60%.The stress of the 12 Mn round billet at different temperatures increases firstly with the strain and then decreases,and finally tends to be stable.Finally,the precipitation phase of the III brittle zone was verified by Thermal-cal software.