The cluster model and the self-consistent extended Huckel method are used to calculate the stable configuration for the chemisorption of Al on both the Si(lll) and GaAs(110) surfaces. The calculated results show that the threefold ionic chemisorption for Al/Si (111) is more stable than that for the covalent top site, and the LDOS for the threefold site agrees quite well with the experimental data. In the case of Al/GaAs(110), the Al adatom substitutes the surface Ga atom to form an AlAs layer, and the Al chemisorption leads the relaxed GaAs (110) back to the unrelaxed ideal position.