A heat exchanger is provided in an air conditioner, refrigerator or the like, and is provided for the indirect transmission and reception of heat between fluids. The heat exchanger comprises a plurality of flat plate fins which are arranged parallel to one another at predetermined intervals so as to allow air stream to flow therebetween, and a plurality of heat transfer tubes passing through the flat plate fins for allowing a fluid to flow therethrough. Each fin is provided with a number of groups of air vents arranged to cross the wake side of the air stream. Each of the air vents is defined by a slat having four sides in which two sides, facing the air stream, are opened and other two sides are provided with leg portions for connecting the slat with the fin. The leg portions are aligned with each other and are inclined with respect to the normal line of the leading edge of the fin. With the above structure, a swirling and a turbulent component of air are induced in the air stream flowing between the fins thereby improving the heat transfer efficiency of the fins.