The chemisorption of Al on the GaAs(ll0) surface is studied by using the charge self-consistent EHMO method. In comparing the two different mechanisms of adsorption, the more stable mechanism is determined by the configuration which gives a lower energy, and it is suggested from the present study that Al will substitute the surface Ga atoms, while the latter will be adsorbed on the dangling bonds of the surface As atoms. The charge transfer, the bond strengths and the density of states are also considered.