A heat exchanger comprises a base portion and a plurality of fins. The fins are disposed on the base portion in parallel along a processing direction. Each fin has a processing surface far away from the base portion. The processing surfaces sunken to form at least one groove, and the groove extends along a grooving direction which intersects with the processing direction. Furthermore, each fin has two upper valley sides at the groove, and the two upper valley sides connect the processing surface. The two upper valley sides and the processing surface form an obtuse angle. The structure of the heat exchanger can prevent processing waste filling in the passages between fins.