In combining with X-ray photoelectron spectroscopy (XPS) measurement, the high resolution electron energy loss spectroscopy (HREELS) is used to investigate the evolution of energy gap width during the formation of Al-GaAs(l00) Interface. It is found that at room temperature, the product of Al-GaAs(100) interface reaction is AlAs. And AlGaAs alloy is formed after an annealing treatment. Low energy electron diffraction (LEED) is used also to study the deposition process of Al on GaAs(100)(4×1) surface. The deposited surface is disordered at the beginning and become ordered with Al deposition amount increasing.