The invention relates with the feed-back control system of a heat exchanger by use of the flow resonance phenomenon which maximizes the heat transfer efficiency by generating the flow disturbances of a heat transfer medium. If a heat transfer medium is periodically stimulated at the characteristic frequency of a heat exchanger, the flow resonance frequency, which is dependent upon the flow conditions of heat transfer medium and the geometries of a heat exchanger, the disturbances of heat transfer medium is increased to the extent that heat transfer is dramatically high. This system is composed of the detecting part for the detection of flow characteristics, the processing part for the determination of flow resonance frequency, and the stimulating part which excites a heat transfer medium at the calculated flow resonance frequency.