A heat exchanger includes a plurality of tubes through which refrigerants flow, the tubes being spaced apart from each other, and a plurality of fins spaced apart from each other at a predetermined distance. Each of the fins has fin collars through which the tubes are perpendicularly inserted, seat portions concentrically formed around outer circumferences of the fin collars and provided with laterally-opened front and rear portions, more than two peak portions, and more than two valley portions, the peak and valley portions being alternately disposed to provide airflow variation.