Provided is a heat exchanger including a core portion configured to include an inlet header tank and an outlet header tank having a space, in which cooling water is stored and flows, formed therein and formed in a height direction, a plurality of tubes having both ends connected to the header tanks to form a cooling water channel, and fins interposed between the tubes, in which a core portion of the heater exchanger is formed to block only a part of a bypass area of a part where inlet/outlet header tanks are positioned to reduce a pressure loss of air which is a cooled fluid while minimizing a reduction in heat radiation performance of the heat exchanger and structural rigidity of the core portion is increased by a reinforcing structure formed at an outer side of the core portion to improve durability.