Abstract: The effects of tempering temperature on the microstructure and mechanical properties of a medium carbon low alloy steel for aerospace applications were investigated by tensile, hardness, impact tests, metallographic and scanning electron microscopy. The lath martensite was formed in the studied steel after quenching. After tempering at 200℃ and 400℃, tempered martensite and tempered troostite were formed respectively. After tempering at 550~650℃, tempered sorbite was formed. With the increase of tempering temperature, the strength and hardness of the studied steel decreased continuously. After tempering at 200℃, 400℃ and 550℃, the studied steel showed obvious intergranular brittle fracture. When the tempering temperature was increased to 600℃ and above, the studied steel showed ductile fracture, and the tensile plasticity and impact toughness were significantly improved. However, the tensile strength and plasticity after tempering at 650℃ were lower than those after tempering at 600℃. After tempering at 600℃, the studied steel obtained a good combination of strength and toughness, with tensile strength reaching 1000MPa level, yield strength reaching 900MPa, hardness reaching 290HBW, elongation after fracture reaching more than 13%, reduction of area reaching more than 21%, and impact absorbed energy reaching 40J.