Supercritical cycles for geothermal power generation systems are studied to raise the power output and thermal efficiency by selecting natural fluids and new organic working fluids as the working fluids and optimizing the cyclic parameters, especially the condensing temperature or pressure. For a given liquid dominated geothermal resource, thermodynamic parameters, using propane, R-125 and R-134a as the working fluids, respectively, are calculated to show the features of supercritical power cycles and compare to other design results shown in references. Greater power output shows that propane and R-134a are appropriate working fluids of supercritical cycles for geothermal binary design.