Initial oxidation process of Mg films was investigated by Anger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A polycrystalline Mg film was exposed to oxygen at room temperature. An inspection of EELS reveals that the initial oxidation process could be classified according to the curve profile in the low energy loss region. The oxide formation is initiated after oxygen exposure of 3 L (Langmuir).