The automatic fuel oil supply unit is composed of oil supply pumps, circulating pumps, flowmeter, mixing tank, primary fuel filter, self cleaning fuel filter, fuel oil heater, fuel oil viscosity control system, PLC electric controller, instruments, valves, attachments, and so on. The heavy fuel oil is heated to more than 80¡æ in the daily oil tank and that enters the inlet of fuel oil supply unit, by primary fuel filter. Then the fuel oil is supplied to mixing tank by supply pumps. By mixture with main engine return oil, circulating pump has increased the pressure to the heater to reduce the viscosity. The self cleaning filter finishes the filtering. Then the heavy fuel oil heating temperature is controlled by viscosity control system, and the fuel oil fitting for main engine enters the main engine.