In this paper after thermo-economic modeling of a solar heater using flat plate collector, the system is optimized. For this purpose, six variables are selected as decision variables. Then Particle Swarm Optimization algorithm is applied to obtain the maximum efficiency and the minimum of the total annual cost as two objective functions. The results of optimal designs are a set of optimum points, named Pareto front. The trend of variation in both efficiency and total annual cost with change in decision variables is also performed and the results are discussed. Mass flow rate as well as insulator thickness are found as two important decision variables which cause a confliction between objective functions. On the other hand, collector length has no effect on the confliction. To generalize the optimum results, the optimization of the system is performed for the different rates of heat transfer. The results reveal that, the efficiency is relatively changed for 0.123%, -0.740% and -1.506%, respectively in the cases of 0.5 kW, 1.5 kW and 2 kW compared with the case of 1 kW. The above changes for the total annual cost are obtained 44.30%, -40.98% and -77.87% which verify the thermoeconomic improvement in the lower rate of heat transfer. (C) 2017 Elsevier Ltd. All rights reserved.