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Power generation systems by renewable energy can reduce primary energy consumption and harmful emissions. Organic Rankine Cycles (ORC), together with solar energy as a heat source, is a possible solution for producing power. In these cycles water is replaced with organic working fluids. This study uses a thermodynamic simulation for performance analysis of ORC, choosing a suitable working fluid for these Cycles. In this study, Parabolic Trough collector as a heat source with temperature of 260°C has been considered. The system defined generated 10kW of electrical power. The effects of the properties of twelve different working fluids, on the overall cycle efficiency have been investigated. The thermodynamic and physical properties, environmental impacts, safety, availability and cost are among the important considerations when selecting a working fluid. N-decane presents the best cycle efficiency, resulting in the minimum collector surface area. However, this fluid has the highest saturated vapor specific volume (resulting in a larger condenser) and the lowest condenser saturation pressure (higher infiltration of non-condensable gases). The best candidates for the cycle regarding all the considered aspects considered are: R113, R123 and Isopentane. Comparing these three fluids, R113 presents the highest efficiency and with highest impact in environment but good fluid from safety point of view. Isopentane presents the smallest cycle efficiency and biggest vapor specific volume, but is the fluid with super-atmospheric saturation pressure and higher flammability. R123 presents the high efficiency and with low impact in environment.