Disclosed is a secondary battery. The secondary battery includes an electrode assembly including a first plate, a second plate, and a separator disposed between the first and second plates, a battery case accommodating the electrode assembly and an electrolyte, a cap assembly sealing an opening formed on one side of the battery case, and a gasket disposed between the cap assembly and the battery case. The battery case includes a crimping part formed on an upper portion of the battery case, and the crimping part is disposed on the gasket. A ratio of a second thickness of the gasket to a first thickness of the gasket is greater than 1.8 and is less than 4.8. The first thickness is a length between an internal surface of the battery case and an outermost edge of the cap assembly, and the second thickness is a length between the internal surface of the battery case and an outermost tip of the crimping part.