Abstract: The lightweighting of automobile structure is the most effective way to realize energy saving and environmental protection of automobile. Heat treatment test of 780DP high strength automobile steel was carried out by tube resistance furnace, the effects of isothermal temperature and isothermal time on the microstructure evolution and Vickers hardness of steel were studied. The results show that the matrix structure only recovers and accompanies by the transformation of ferrite to austenite at 780-840℃ for 10-300 s isothermal heat treatment. With the prolongation of holding time and the increase of isothermal temperature, the volume fraction of martensite increases continuously. With the prolongation of holding time, the Vickers hardness of the experimental steel decreases first and then increases due to the change of the recovery and martensite volume fraction during isothermal process.