Auger Electron Spectroscopy was used to characterize the initial stages of the oxidation of Ce-5% La alloy in an oxygen atmosphere at low pressure( ～ 10-6 Pa) and room temperature after the surface is cleaned by Ar+ ion bombardment.It is shown that exposure of clean cerium to oxygen causes the appearance and development of three new Auger peaks at 97,662 and 676 eV, which steadily grow during oxidation of cerium.Upon oxygen dose less than 20 L,a semi-protective layer of oxide forms on the surface of cerium and its growth follows a logarithmic relationship.With further exposure of oxygen, the oxide film grown in the previous stage becomes thicker and the uptake of oxygen reaches saturation at oxygen exposure of 25 L,and the oxide film mainly consists of Ce2O3.