Abstract: The effects of two different processes, normalizing with air cooling + tempering and normalizing with controlled cooling+ tempering, on the retained austenite and mechanical properties of medium-carbon low-alloy martensitic ultra-high strength steel were investigated to improve its plasticity and toughness. The microstructure of the steel was observed by scanning electron microscopy (SEM). The volume fraction, shape and distribution of retained austenite in the steel were measured by X-ray diffraction (XRD) and electron back-scattering diffraction (EBSD). The results show that lath martensite and retained austenite (M+RA) are obtained by the two processes and a uniform distribution of retained austenite is located between lath martensites. The volume fraction of retained austenite varies from 3% to 10% with treatment parameters. Normalizing with controlled cooling can significantly refine lath martensite while the yield strength and tensile strength increase more than 100 MPa and the impact energy decreases by 4 J. When the quenching temperature rises, the volume fraction of retained martensite increases firstly and then decreases. Normalizing with controlled cooling can also act as a method for improving the microstructure and properties of martensitic steel.