The dependence of microstructures and mechanical properties on tempering temperature (from 180 to 650 °C) in a designed 3Mn-Si-Ni martensitic steel was systematically analyzed. Microstructure was characterized using scanning and transmission electron microscopy; mechanical properties were measured using uniaxial tensile test and Charpy V-notch impact test. After tempering at different temperatures, recovery, partial recrystallization, carbides precipitation and decomposition of residual austenite were observed. After tempering at 230 °C, an excellent combination of strength (1550 MPa) and toughness (91.5 J) was achieved, due to high dislocation density and ε-carbides precipitation. However, with an increase in tempering temperature from 320 to 550 °C, tempered martensite embrittlement was observed, where impact energy was ~ 10 J. It was ascribed to cementite formation instead of transition carbides and decomposition of residual austenite. With an increase in tempering temperature up to 650 °C, high fracture impact toughness of 75 J was obtained with deteriorated tensile strength of 850 MPa due to strong recovery and partial recrystallization.