The early oxidation process of pure Fe was studied by XPS, UPS and AES. When the temperature and partial pressure of oxygen is not too high, the product of the reaction between polycrystalline Fe and oxygen is pure Fe3O4, but not FeO or a-Fe2O3 or a structure of layers of the three compounds. If the temperature is higher than 600℃ and the partial pressure of oxygen approach 1 × 10-6 Torr, the thermo-desorption process occurs instead of oxidation process on polystalline Fe surface.