The differences of grain-refining effect between Sc and Ti additions in aluminum, which cannot be substantially explained by traditional theories, were carefully studied. The empirical electron theory (EET) of solids and molecules was employed to calculate the valence electron structures (VES) of Al3Ti and Al3Sc. The conclusions can be drawn that, in the two alloys Al-Ti and Al-Sc, the different valence electron structures of Al3Ti and Al3Sc and the consequent differences of growth habit of the two particles, and the different interfacial electron density between particles and matrix fundamentally lead to the differences of grain-refining effect between Sc and Ti additions on aluminum and make Sr the better grain-refiner of aluminum.