The experimental method for studying interface atomic structure between amorphous layer and crystal by MeV ion scattering and channeling is described. This method is applied to study the Al_2_O_3_/AI(100) interface . Thin AI oxide film are formed on clean AI(100) surface in pure oxygen ambience at 400℃. The experimental results show that the composition of the oxide film is precisely Al_2_O_3 and the interface between AI oxide film Al(100) bulk is very sharp, the reconstructive layer under the Al_2_O_3 film is no more than one monolayer. Monte carlo simulations indicate the distance between displaced Al atoms on the reconstructive layer and Al(100) axis is 0.18A. At room temperature, the oxygen adsorption layer of Al(100) is primarily stoichiometric AlO at low surface coverages.