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The effect of heat treatments was investigated on the microstructure evolution and mechanical properties of X12CrMoWVNbN10-1-1 steel. The as-received steel was austenitized at 1080 °C for 16 h and cooled in furnace, following tempered in two steps, i.e. at 570 °C for 18 h and then at 690 °C for 24 h. The austenitized sample had a typical lath martensite structure with some retained austenite and no delta ferrite was observed after austenitization. After the first tempering at 570 °C, there was no retained austenite remained but it resulted in the forming of precipitates, such as Cr-rich M7C3, Cr-rich M23C6, Cr-rich M2N, Nb-rich MN, and the subgrain, as well as the reduction in dislocation density. However, microstructure observation after secondary tempering proved the further arrangement of dislocation, subgrain growth and the coarsening of precipitates. Hardness and impact tests were carried out for establishing a correlation between microstructure and mechanical properties. Finally, a careful fractographic analysis of impact samples had been done using SEM and EDS.