In this paper, a practical method for designing Organic Rankine Cycle (ORC) power generation system was proposed. The design method was aimed to optimize the cost performance of ORC system and based on experiments and theoretically modeling. In this study, two experimental systems have been built. The first experimental system was used to offer data which were essential to establish the relationships needed to build the semi-empirical model. The second experimental system was employed to validate the model. In the semi-empirical model, objective function was established to quantify the ratio of power output to heat exchangers' area (preheater, evaporator and condenser's area), which was used to characterize the cost performance of ORC systems. By combining the experimental data from the first experimental system and theoretical formulas, the objective function was derived with a single variable: evaporation temperature. In the objective function, the optimized evaporation temperature could be obtained by using extremum method. Then other parameters of the ORC system could be also acquired through the model which would allow the ORC system to have good or the best cost performance. (C) 2015 Elsevier Ltd. All rights reserved.