Indonesia, being one of the world's biggest producer of palm oil, has the potential to utilize palm oil as a replacement for fossil fuel. However, palm oil possesses a high level of viscosity. A heater system is needed to reduce the viscosity of palm oil as well as switching of fuel channel between diesel oil and mixed oil (palm oil and diesel oil). The purpose of this study is to design an automatic heating system for use on a small-scale diesel power plant. The automatic heating and switching system consist of a heater element, microcontroller, temperature sensor, relay and solenoid valve. This system read the temperature of mixed oil. If the temperature of mixed oil is below the temperature limit that is specified on the microcontroller then the heater will turn on and the solenoid valve will allow the diesel to flow to the engine. On the other hand, if the temperature of mixed oil is above the temperature limit that is specified on the microcontroller then the heater will turn on and the solenoid valve will allow the mixed oil to flow to the engine. The result of this study is this system can maintain a stable mixed oil temperature in the range specified on the program executed by the microcontroller with a low deviation value. The performance analysis of this study hopefully can be a reference for future studies on the usage of automatic mixed oil heater for small-scale diesel power plant.