A device known as Real-Time Non-Surfactant Emulsion Fuel Supply System (RTES) was developed to eliminate the dependency of surfactant by rapidly supply the water in diesel (W/D) emulsion into the engine. The objective of this research is to run RTES under long term of operation. A new RTES was fabricated using ultrasonic transducer frequency is 50 kHz and speed of rotor is 1400 rpm. The amount of water injected into the system was constantly at 6.5 vol%. At the end of the test, temperature measurement and observation of RTES component were recorded and droplet size before and during the durability test was measured. It can be concluded that RTES can only withstand the long-term operation for total 26 hours. On the droplet size measurement, initial droplet size is 1.441µm, as the temperature of ultrasonic transducer increase, the droplet size also increases.