The dilatometer curves of continuous cooling transformation of 12Mn steel were measured with Formastor-FⅡthermal mechanical simulator.The steel's undercooled austenite phase continuous cooling transformation curves ( CCT curves) were established by means of the dilatometer method and the metallographic-hardness measurement method. The effect of cooling rate on microstructure and hardness of the steel was studied. CCT curve of test steel was simulated by JMatPro. The results show that the Ac1 and Ac3 of the experimental steel are 692 ℃ and 855 ℃ ; the microstructure obtained is made of ferrite,pearlite and bainite.The ferrite transformation and pearlite transformation occur at a slower cooling rate,in which the ferrite is dominant. When the cooling rate is greater than 4.25 ℃ / s bainite transformation happens.As the cooling rate increases,microstructure or grains become finer.The hardness of the tested steel with increasing cooling rate shows a trend of first fast increase and soon decrease.The simulation results are consistent with the measured CCT law. The high temperature mechanical properties of 12mn steel round billet were tested by gleeble-1500d thermal / mechanical simulator. The tensile strength, reduction of area and stress-strain curves of the billet were obtained in the range of 600-1300 ℃.