This study researches cold rolled ultra high strength martensitic steel processed by water quenching.It is found that both the quenching and overageing temperatures greatly influence the mechanical properties of martensitic steel.A tensile strength of 1 500 MPa can be obtained from 0.2%C-1.8% Mn steel by soaking at 840 ℃,quenching at 700 ℃ and overageing at 200 ℃ for several minutes.The continuous cooling transformation(CCT) diagram reveals that full martensite can be obtained at a cooling rate of 100 ℃/s or higher;and at a cooling rate of 3-10 ℃/s,austenite barely decomposes at 700 ℃.For steel with 0.2% carbon and less manganese,austenite decomposition occurs before it is cooled to 700 ℃ at a cooling rate of 3-10 ℃/s,which leads to lower tensile strength.It is possible to reduce the manganese content of the 1 500 MPa martensitic steel by increasing the quenching temperature.To increase the quenching temperature,the control of flatness during water quenching becomes a major concern. This study researches cold rolled ultra high strength martensitic steel processed by water quenching. It is found that both of quenching and overageing temperatures greatly affect the mechanical properties of martensitic steel. A tensile strength of 1 500 MPa can be obtained from 0.2% C-1.8 % Mn steel by soaking at 840 ° C, quenching at 700 ° C and overageing at 200 ° C for several minutes. Continuous cooling transformation (CCT) diagram shows that full martensite can be obtained at a cooling rate of 100 ° C / s or higher; and at a cooling rate of 3-10 ° C./s, austenite barely decomposes at 700 ° C. For steel with 0.2% carbon and less manganese, austenite decomposition occurs before it is cooled to 700 ° C. at a cooling rate of 3-10 ° C./s , which leads to lower tensile strength. It is possible to reduce the manganese content of the 1 500 MPa martensitic steel by increasing the quenching temperature. To increase the quenching temperature, the control of flatness during water quenching becomes am ajor concern