A header unit having an improved assembly method of a body and a header and a heat exchanger having the same, capable of preventing a body from being incompletely bonded to a cover at an inner side of the header as a result of the instability of a manufacturing process, and thus capable of preventing the leaking of the refrigerant in between two tanks at an inside of the header. The heat exchanger includes a body, and a cover coupled to the body, wherein the body includes a base part forming a bottom surface of the body, and a middle partition protrudedly formed from the base part, and the cover includes a coupling groove to which the middle partition of the body is coupled while passing through the coupling groove.