Effects of wavy-fins surface area on thermal-hydraulic performance of a heat exchanger have been observed. First, a new method to calculate the heat transfer area of wavy-fin surfaces is introduced. The results show that the proposed method is accurate enough to be used in the analysis of heat exchanger performance. One of the important aspects of this method is that it is a direct method compared with the experimental method introduced by Kays and London, and thus might be a strong tool in the optimization of heat exchangers based on different objective functions. Effects of some nondimensional parameters, such as amplitude-to-wavelength ratio, fin space ratio, and channel cross-section ratio on the heat transfer characteristics and pressure drop are also investigated.