Pt/CoOx/Pt tri-layers exhibited reproducible and stable unipolar switching under a dc sweeping voltage. In order to investigate the role of oxygen reduction in the metal-deficient CoOx layer, resistive switching of the post-annealed CoOx thin film was compared with those of the as-deposited CoO thin film. X-ray photoemission spectroscopy showed larger reproducible resistance switching and decreasing of current level in the post-annealed CoO thin film. This may be explained by a reduction in oxygen stoichiometry without phase transformation of the CoOx. In addition, stable switching in post-annealed CoOx layer is considered to originate from the decrease of the Co vacancies in local Co3O4 region partially distributed in the whole CoOx layer, not in the dominant CoO. (C) 2010 Elsevier B.V. All rights reserved.