A heat exchanger that effectively distributes a refrigerant by varying the cross-sectional area of a header and an air conditioner having the same. The heat exchanger includes a plurality of refrigerant tubes disposed spaced apart from each other, a header joined to both ends of each of the refrigerant tubes, at least one baffle to divide the refrigerant tubes into a plurality of mutually adjacent groups and block a longitudinal flow of a refrigerant flowing in the header, each of the groups causing the refrigerant to flow in one direction, and a booster installed to vary a cross-sectional area of the header to uniformly distribute the refrigerant to refrigerant tubes in the same group among the refrigerant tubes. The booster to vary the cross-sectional area of the header may allow a refrigerant flowing through a header to be effectively distributed to the refrigerant tubes.