Abstract: The on-line quenching and 600 ℃ tempering and the out-line quenching and 600 ℃ tempering treatments were conducted on a high-strength steel plate. The microstructure and mechanical properties by the two processes were studied and compared. The results show that the microstructures of the test steel plate under the two processes were both tempered martensite. After the on-line quenching and tempering treatment, the martensite laths had smaller spacing and longer length. Moreover, the martensite after the on-line quenching and tempering had a higher proportion of dislocation density and more large-angle grain boundaries, and the carbides were finer. The mechanical properties of the test steel plate after the on-line quenching and tempering treatment showed higher strength, but the plasticity and toughness were lower than those after the out-line quenching and tempering treatment.