Oxidation of transition metals has been a practical problem of corrosion. However the microscopic mechanism of the oxidation of transition metals, at high temperatures in particular, has not been investigated in detail. In the present study the mechanism of the oxidation of zirconium at high temperatures has been clarified by investigating the oxidation states at the zirconium surface by the use of secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), secondary electron emission spectroscopy (SES) and the work function method. The oxidation of zirconium has been shown to go through the three stages: the adsorption of oxygen, nucleation of oxide and growth of oxide.