A secondary battery which can improve safety is provided. The secondary battery includes an electrode assembly, a can accommodating the electrode assembly and a cap assembly coupled to a top portion of the can, wherein the cap assembly comprises a cap-up, a safety vent installed under the cap-up, a cap-down installed under the safety vent, an insulator interposed between the safety vent and the cap-down, and a sub-plate positioned on a bottom surface of the cap-down, and an insulation layer is formed on a top surface of the cap-down.