A battery pack includes a bare cell having a pair of first side portions, a pair of second side portions, and a pair of plane portions, each of which are opposite to each other. The second side portions and the plane portions are connected to ends of the first side portions. A protection circuit module is electrically connected to the bare cell. An inner frame exposes one side portion of the pair of first side portions and accommodates the bare cell. An outer frame exposes the pair of plane portions and accommodates the bare cell, the protection circuit module and the inner frame. In the battery pack, outer ribs extending away from the bare cell are further formed at the inner frame that comes in contact with the pair of second side portions of the bare cell so that the outer frame is surrounded by the outer ribs.