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A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m 3 was observed from ultrathin Fe/MgO (001) bilayers grown on Cr-buffered MgO (001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m 3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.