This study aims to investigate the effect of the 1-step quenching and partitioning( Q&P) process on the microstructure and the resulting Vicker’s hardness of 0. 3C-1. 5Si-1. 5M n steel by using in-situ dilatometry,optical microscopy( OM),scanning electron microscopy( SEM),X-ray diffractometry( XRD),and Vicker ’s hardness measurement. Systematic analyses indicate that the microstructure of the specimens quenched and partitioned at150 ℃,200 ℃,250 ℃,and 300 ℃ mainly comprises lath martensite and retained austenite. The dilatometry curve of the specimen partitioned at 150 ℃ is presumably ascribed to the formation of isothermal martensite. In the early stages of partitioning at 200 ℃,the nearly unchanged dilatation curve is closely related to the synergistic effect of isothermal martensite formation and transitional epsilon carbide precipitation. In the later stages of partitioning at200 ℃,the slight increase in the dilatation curve is due to the continuous isothermal martensite formation. With further increase in partitioning temperature to 250 ℃,the dilatation increases gradually up to 3600 s,which is related to carbon partitioning and lower bainite formation. Partitioning at a higher temperature of 300 ℃ causes a rapid increase in the dilatation curve during the initial stages,which subsequently levels off upon prolonging the partitioning time. This is mainly attributed to the rapid diffusion of carbon from athermal martensite to retained austenite and continuous formation of lower bainite. This study aims to investigate the effect of the 1-step quenching and partitioning (Q & P) process on the microstructure and the resulting Vicker’s hardness of 0. 3C-1. 5Si-1.5Mn steel by using in situ dilatometry, optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD), and Vicker’s hardness measurement. Systematic analyzes indicate that the microstructure of the specimens quenched and partitioned at 150 ° C, 200 ° C, 250 ° C, and 300 ° C mainly comprises lath martensite and retained austenite. The dilatometry curve of the specimen partitioned at 150 ° C is presumably ascribed to the formation of isothermal martensite. In the early stages of partitioning at 200 ° C, the nearly unchanged dilatation curve is closely related to the synergistic effect of the isothermal martensite formation and transitional epsilon carbide precipitation. In the later stages of partitioning at 200 ° C, the slight increase in the dilatation curve is due to the continuous isothermal martensi te formation. With further increase in partitioning temperature to 250 ° C, the dilatation increases gradually up to 3600 s, which is related to carbon partitioning and lower bainite formation. Partitioning at a higher temperature of 300 ° C causes a rapid increase in the dilatation curve during the initial stages, which subsequently levels off upon prolonging the partitioning time. This is mainly attributed to the rapid diffusion of carbon from athermal martensite to an existing austenite and continuous formation of lower bainite.