Mechanical stress is generated in electrode particles of a Lithium ion Polymer Battery (LiPB) during charge and discharge. The stress can cause cracks and fractures in the solid particles over time when cells are cycled, which leads to a disorder and a fracture of the electrodes. In order to understand the mechanism, a stress model for a pouch type high power LiPB is developed based on electrochemical and thermal model, where the stress induced by the ion concentration in the electrode particles is considered. The stress in the particles causes changes in the electrode volumes and leads to changes in the thickness of a battery cell, which is measured using a device designed with two linear voltage displacement transducers (LVDT). This model is validated against experimental data obtained from a pouch cell. Analysis shows that the magnitude of stress depends upon locations and C rate, while the thickness of a single cell is mainly affected by the state of charge (SOC), but not C rate.