In this study, the 38MnSiVS medium carbon non-quenched and tempered steel bar was investigated by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and quantitative metallography. The microstructures were mainly composed of ferrite and pearlite. The phase transition-microstructure prediction models were established to calculate ferrite volume fraction fα, ferrite grain size dα and pearlite interlamellar spacing So. The volume fraction of the second phase V (C, N) was calculated by thermodynamics. The morphology of the second phase V (C, N) was observed by TEM, and the distribution of particle size of the second phase was determined. And the mechanical properties were measured. Considering the microstructural parameters (fα,dα and So), the modified coefficient of solid solution elements, and the effect of precipitates, combining with some reference models, prediction model of mechanical properties including yield strength, tensile strength and impact toughness were finally established.