An Organic Rankine Cycle for diesel engine waste heat recovery is modeled and optimized. The design parameters are nominal capacity of diesel engine, diesel operating partial load, evaporator pressure, condenser pressure and refrigerant mass flow rate. In addition four refrigerants including R123, R134a, R245fa and R22 are selected and studied as working fluids. Then, the fast and elitist NSGA-II (Non-dominated Sorting Genetic Algorithm) is applied to maximize the thermal efficiency and minimize the total annual cost (sum of investment cost, fuel cost and environmental cost) simultaneously. The results of the optimal design are a set of multiple optimum solutions, called Pareto optimal solutions. The optimization results show that the best working fluid is R123 in both of economical and thermo dynamical view point for a specified value of output power. R245fa, R134a and R22 are placed in the next ranking, respectively. The optimum result of R123 shows the 0.01%, 4.39%, and 4.49% improvement for the total annual cost in comparison with R245fa, R22, and R134a, respectively. The above values for efficiency are obtained 1.01%, 12.79% and 10.57%, respectively. Furthermore R123 needs the highest investment cost while the environmental and fuel costs are the lowest. (C) 2013 Elsevier Ltd. All rights reserved.