According to the empirical electron theory of solids and molecules, the valence electron structures of GP zone, θ″, θ′ and θ phases in Al-Cu alloys are calculated out. The results show that the number of covalent electron pairs on the strongest bond A of GP zone is greater than that of of the matrix, i.e., the obstruction of in GP zone for dislocation movement is stronger than that of in the matrix. Only based on the bond strength of composition atoms, it can be found, , , and phases have strengthening effects for Al-Cu alloys. The strengthening effect of is the strongest, and is weaker, and is the weakest. Compared with, the bond strength of and is weaker than that of . From the value of nA, it is easy to reconstruct the bonds of GP zone, , and , but is difficult to reconstruct the bonds of .