In this study, the fin effectiveness method is employed to investigate the thermal characteristics of parallel flow, counter flow and cross flow heat exchangers operated in series. The specific heat of the fluids and overall heat transfer coefficient are assumed constants. In the beginning, analysis is focused on the minimum heat transfer areas of heat exchangers. The temperature of the fluid flow in amid of the exchangers then is obtained. Subsequently, the design method is put into some special practical applications. From the calculated results, the optimum operating conditions of parallel flow, counter flow and cross flow heat exchangers are computed and proposed respectively.