Pt/CoO x /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 CoO x layer, resistive switching of the post-annealed CoO x thin film was compared with those of the as-deposited CoO x thin film. X-ray photoemission spectroscopy showed larger reproducible resistance switching and decreasing of current level in the post-annealed CoO x thin film. This may be explained by a reduction in oxygen stoichiometry without phase transformation of the CoO x . In addition, stable switching in post-annealed CoO x layer is considered to originate from the decrease of the Co vacancies in local Co 3 O 4 region partially distributed in the whole CoO x layer, not in the dominant CoO.