Abstract: High pressure heat treatment was conducted on cold-rolled EN950C building steel plates at different temperatures （700, 750, 800, 850, 900 ℃） for 30 min and at 750 ℃ for different durations （5, 10, 30 min）. The effects of temperature and holding time on the microstructure and mechanical properties of the steel plates were studied. The results show that after high pressure heat treatment under different conditions, the steel plate still retained its original fibrous structure characteristics, but the white granular second phases were precipitated in the structure. With the increase of the high pressure heat treatment temperature, the number of granular second phases decreased and the second phases distributed from along the rolling direction to along the grain boundaries. With increasing holding time, the number of granular second phases decreased and the size of granular second phases and of the grains of the matrix increased. Compared with those of the cold-rolled steel plates, the hardness, tensile strength and yield strength of the steel plates after high pressure heat treatment decreased, while the percentage elongation after fracture increased. With the increase of the heat treatment temperature, the hardness of the steel plates decreased rapidly and then stabilized, the tensile strength, yield strength and ratio of yield to strength decreased, and the percentage elongation after fracture increased. With the increase of holding time, the hardness, tensile strength, yield strength and ratio of yield to strength of the steel plates decreased, and the percentage elongation after fracture increased.