A magnetic tunnel junction (MTJ) with a CoFeB/MgO structure demonstrated a high tunnel magnetoresistance (TMR) ratio with a perpendicular magnetic easy axis1, and has been investigated toward an integration in VLSI2 to replace a conventional volatile cache memory with a magnetic random access memory3. A MgO tunnel barrier has been formed on a CoFeB film by a magnetron sputtering of MgO target4 or by a natural oxidation of Mg film5 to form a CoFeB/MgO stack. In both cases, the surface of the CoFeB film underneath has been found to be oxidized during MgO deposition process4,5 and to affect an interface perpendicular magnetic anisotropy5. A reduction of the oxidized Fe at the MgO/CoFeB interface was necessary to achieve a high TMR ratio6. In this experiment, a reduction reaction of oxidized Fe/Co by B on the naturally oxidized surface of the CoFeB film after an annealing has been investigated using X-ray photoelectron spectroscopy (XPS)7. In this work, we focused on the diffusion behaviors of the constituent elements at the CoFeB film surface after the natural oxidation and the annealing process to elucidate the oxidation and reduction mechanisms of the CoFeB film.