The characteristic temperature in the definition of the fin side surface heat transfer coefficient of two tube bank fin heat exchangers is investigated in detail. The results reveal that if the average temperature of the fin side fluid is used in the definition of the fin side surface coefficient, a small discrepancy between the numerical and experimental results is obtained. The results not only give a technical definition of the characteristic temperature of the fin side surface heat transfer coefficient, but also imply that the discrepancy between experimental and numerical results may come from incorrect use of the characteristic temperature.