The high demand of electricity and the diverse of heat source made the researcher do a modification on a power plant. One of the modifications is Organic Rankine Cycle (ORC) system, where the system uses a low heat source. In addition, the system uses refrigerant as a working fluid instead of water. However, the halocarbon refrigerant has a negative impact on environment since they have a high number of Global Warming Potential (GWP) and Ozone Depletion Potential (ODP). The natural refrigerant is an alternative from the halocarbon refrigerant, with the low of GWP and ODP. Mixing hydrocarbon, like propane and butane, would improve the heat transfer characteristic. Musicool-134 (MC-134) is a mixture refrigerant with the main substance is between propane and i-butane. Based on the design calculation, the ORC system with MC-134 as a working fluid would result around 206.76 kW of work from the turbine and the thermal efficiency about 40%. This study aims for suggesting a solutions of ORC systems with a mixture of propane and i-butane as a working fluid to be used as a small to medium power plant.