Abstract: A novel ultrasonic wear debris sensor is designed. The conformity of sensor output as well as the influence of oil temperature and viscosity on the output was analyzed experimentally. It is revealed that the ultrasonic wear debris sensor can detect wear debris in oil as small as 45 μm and had a good consistency. The output of the sensor decreased with the increase of oil temperature and viscosity. The performance degradation of the transducer due to high temperature was evident. The acoustic absorption in the high-viscosity oil at low temperature was the main reason of the decrease of sensor output. The correction methods for the wear debris position in detecting zone and the oil temperature and viscosity were proposed.