All-solid state Li ion batteries are expected as one of next-generation rechargeable batteries since they have the potential to achieve high energy density and are free of risk of a flammable liquid electrolyte. The Li ion batteries cathodes, such as LiCoO2, change their volume during insertion or extraction of Li ions. Therefore, mechanical stress can be induced at the electrode/electrolyte interface. This mechanical stress can affect the thermodynamic and/or kinetic properties of electrodes and electrolytes. In this study, we aimed to understand the influence of the mechanical stress on Li chemical potential of LiCoO2. The variation of Li chemical potential in LiCoO2 under mechanical stress was evaluated by electromotive force (emf) measurements. Positive emf was observed when the external mechanical stress was applied. This indicates that Li chemical potential in LiCoO2 was changed by the mechanical stress. The relationship between Li chemical potential and the mechanical stress was discussed from thermodynamic point of view.