The selection of appropriate fluid in Organic Rankine Cycles (ORC) is an important issue. The thermodynamic properties of working fluids affect the efficiency, operation and environmental impact of the system. Many working fluids for ORC power plants are available. Pentane (R-601) and R245fa are the most used for ORC. In this work, a gene expression programming (GEP) model for estimating thermodynamic properties of pentane (R-601) and R245fa was used. New formulations are presented for determination of the enthalpy and entropy values of two working fluids. The actual results were compared to the results obtained with GEP. The obtained results showed that the formulations are effectively capable of evaluating the thermodynamic properties of working fluids. The GEP-based formulations are simple. These formulations will be helpful for thermodynamic analysis of ORC systems.