Based on the theories of entropy, entransy and exergy, the concepts of entropy generation rate, revised entropy generation number, exergy destruction rate, entransy loss rate, entransy dissipation rate and entransy efficiency are applied to the optimization of the Organic Rankine Cycle. Cycles operating on R123 and N-pentane have been compared in three common cases which are variable evaporation temperature, hot stream temperature and hot stream mass flow rate. The optimization goal is to produce maximum output power. Some numerical analyses and simulations are presented, and the results show that when both the hot and cold stream conditions are fixed, all the entropy principle, the exergy theory, the entransy loss rate and the entransy efficiency are applicable to the optimization of the ORC, while entransy dissipation is not. This conclusion is available no matter what kind of working fluid is used, nevertheless, the system performances and parameters may be much different. The results also indicate that when the hot stream condition (temperature or mass flow rate) varies, the entransy loss rate is the only parameter which always corresponds to the maximum power output. (C) 2014 Elsevier Ltd. All rights reserved.