Abstract: Taking advantage of the Empirical Electron Theory (EET) of solid and molecules, an analysis was made to the valence electron structure of cells either containing or not containing carbon of austenite in Co-free high-strength high-toughness. The effect of those cells containing carbon cells was taken into account preferably in phase transformation process because the value nA of theirs is greater than that of the cells containing no carbon. In the cells containing carbon the alloying elements Ni, Si, Cr and Mo form a gathering borough together with carbon atoms, and the forces binding Ni and Si to C are greater than those binding Cr and Mo to C. The gathering borough will reduce the diffusivity of carbon, hinder dislocation from moving and delay the martensite transformation so as to keep dislocation density high in matrix, thus keeping a certain number of residual austenite in the steel, making austenitic grain size thinner and minimizing the size of martensite transformed. All of the results will benefit improving the toughness of the steel.