Nickel thin films (thickness ; ～1μm) were oxidized by the method what Yoshimura et al called hydrothermal electrochemical synthesis in an aqueous solution of LiOH (O-1N) at high temperature (100-200℃) in an autoclave with the anodic current of 0-100 mA/cm^2 for O-120 min. The reacted specimens were analyzed by FE-SEM, Auger electron spectroscopy, XPS, SIMS and thin-film X-ray diffraction analysis. NiO and Ni(OH)_2 coexisted. Ni(OH_2 were detected mainly near the exposed surface of the films. NiO was remarkably formed at high temperature and high current density in the present experimental conditions. Lithium was detected in the film although no definite conclusion for the formation of solid solution into NiO
