Tempering is an important phenomenon in ferrous alloys. Most steels, especially alloy-ing steels, are used after quenching and tempering. To design the composition of quenching andtempering steels, control the tempering process more effectively, improve the properties aftertempering and realize the potentials of steels, the essence of tempering process and the proper-ties of tempering products have to be understood. In this paper, the phase structure factors andinterface conjunction factors of common alloying elements in cementite and its interface are calcu-lated out. The relationships between these valence electron structure parameters and (i) thephenomena of martensite decomposition, (ii) transformation, gathering and growth of the carbidesand (iii) the mechanical properties of tempering products are built up. The nature of the effect ofalloying elements on tempering process and properties of tempering products is uncovered on thelevel of valence electron structure. One new theoretical foundation for alloy design is provided.