A lithium ion secondary battery having a safety vent responsive to temperature and pressure. The lithium ion secondary battery includes an electrode assembly having a positive electrode plate, a separator, and a negative electrode plate which are simultaneously wound and laminated, and positive and negative electrode leads extending outward from the positive and negative electrode plates, respectively. A can containing the electrode assembly and having an opening; and a cap plate coupled to the opening of the can, wherein an electrode terminal extends through and is coupled to the center of the cap plate with a gasket interposed therein, the negative electrode lead being connected to the electrode terminal, the positive electrode lead being connected to the cap plate, a coupling hole being formed on the cap plate, and a safety vent adapted to soften at a predetermined temperature is coupled to the coupling hole.