As global warming is rapidly emerging as one of the inherently global issues, one of the renewable energies, i.e. thermo-electric generation, has received attention. In order to increase the efficiency of thermo-electric generation, a maximum temperature difference in plain fins in a heat exchanger is needed, and an appropriate pressure drop is required to ensure stable flow of high temperature fluid. In the present study, the characteristics of the temperature difference and pressure difference for the 2 kW-class thermo-electric generator installed in exhaust pipes of a cogeneration plant were investigated numerically via thermal fluid analysis using ANSYS CFX. Then, size optimization for plain fins of a heat exchanger was carried out using SAS JMP, in which the temperature difference was maximized while meeting the requirement of a given pressure drop condition. A meta-model was generated by using the response surface model, and individual desirability functions were defined to derive the optimal solution that provided the maximal overall desirability function. The result obtained by size optimization showed that the temperature difference of the optimized plain fins of a heat exchanger increased by approximately 27% in comparison with the original model under the given pressure drop condition.