Based on the empirical electron theory of solids and molecules(EET) and valence electron theory of composition design of alloy, the valence electron structure(VES) of phases and phase interfaces of ZL203 are calculated in this paper, and the relationship between the VESs and mechanical properties are also studied. The results are as followed: 1) The of GP is bigger than of a, in other words, the resistance of dislocation movement in GP zone is bigger than that of in a matrix. 2) Compared with a matrix, the phases of q¢¢、q¢、q all have strengthening effects. From the bond combination of atoms composed in the strengthening phase of view, the strengthening effect of q is the best, that of q¢ is second, that of q¢¢ is the worst. 3) The precipitation sequence determined by nA is well accordance with that of depended on thermodynamics free energy. 4) The electron density difference Dr of a/GP, a/q" and a/q¢ interfaces increases one by one, and the stress of these interfaces also increases one by one, therefore the strength falls down one by one. 5) Combined with FSFs and ICFs, we can deduce that the best aging stage of ZL203 is the end of the precipitation of q¢¢ and the beginning of the precipitation of q¢.