An on-line monitoring lubricating oil viscosity sensor based on vibration method is developed. The response of the sensor to lubricating oil viscosity, the influence of temperature on the sensor and the stability of the sensor are tested by laboratory and diesel engine bench experiments under constant and variable temperature conditions. The results show that the sensor has high response and sensitivity to the viscosity change of lubricating oil, can accurately measure the viscosity of lubricating oil, and is stable and reliable.