Organic Rankine Cycles are systems used to generate electricity from alternative energy sources (solar energy, geothermal energy and waste heat) under low temperature and pressure. In Organic Rankine Cycles, environmentally friendly refrigerants are used. In this study, geothermal energy which is a rich potential for our country as a heat source for Organic Rankine Cycle (ORC), has been considered and theoretical thermodynamic analysis of the cycle has been made. It is assumed that R-124 fluid has been used in the cycle and analyzed according to different boiler and condenser temperatures in order to examine the effect on the cycle performance. According to the results, thermal efficiency of power cycle increases as boiler temperature increases. Similarly, while the condenser temperature increases, the thermal efficiency of the power cycle decreases. In addition, different organic fluids have been analyzed in order to determine the suitable fluid for the Organic Rankine Cycle in the study. A comparison of the performance, environmental and safety effects of Organic Rankine Cycles obtained by selecting R-134a, R-123, R-124, R-125, R-404A, R-407C, R-410A, R-600, R-600a and R-290 as organic fluid has been performed. Calculations have been made in Engineering Equation Solver (EES) program. From the comparison results, R-123 has the maximum power cycle thermal efficiency while R-125 has the minimum. It has been observed that R-600, R-600a and R-290 fluids are most suitable fluids for the environmental evaluation according to other fluids. However, these fluids need safety precautions, owing to their flammability.