Magnesium oxide (MgO)-modified lithium cobalt oxide (LiCoO2) thin film electrodes were prepared by pulsed laser deposition (PLD) and effects of surface modification by MgO on interfacial reactions of LiCoO2 were studied. The modification by MgO was carried out by PLD on LiCoO2 thin film electrode successively after the deposition of LiCoO2 thin film by PLD. Auger electron spectroscopy suggested that Mg dispersed uniformly in nano-scale on the film electrode. Cyclic voltammetry measurements clearly showed that MgO modification suppresses the increase of resistances caused by repetition of lithium-ion insertion–extraction reaction charged up to 4.2V versus Li/Li+. Moreover, MgO modification decreased the activation energy of lithium-ion transfer reaction at LiCoO2 thin film electrode–electrolyte interface, indicating that the modification by MgO affects the kinetics of lithium-ion transfer reaction at LiCoO2–electrolyte interface.