As compared with traditional offline detection, online oil monitoring technology has the advantages of realtime, synchronization, continuity, and etc. Ultrasonic is of good realtime, high spatial resolution and strong penetration ability in oil, so the development of online ultrasonic wear debris monitoring technology is of great significance. A set of online oil wear debris monitoring system based on the principle of ultrasonic scattering was designed. The hardware system was consisted of ultrasonictesting card, industrial control computer (IPC), ultrasonic sensor, online oil temperature sensor, gear pump and other components. The program of the system was developed by Delphi. A lot of experiments were implemented on the equipment. The results show that the online monitoring system can monitor the number and size of oil wear debris perfectly in near realtime.