Wear changes the contact state and contact stiffness of the interface between the ball and the outer raceway under the operation of a deep groove ball bearing, resulting in the change of reflection coefficient of the interface. This paper describes a method based on ultrasonic reflection to assess the wear status of rolling element bearings. A deep groove ball bearing wear test was performed and the reflected pulses were collected with a linear ultrasonic probe mounted on the bearing outer ring. The results show as the wear intensity first increases and then decreases from running-in stage to steady wear period, the reflection coefficient of the interface between the outer raceway and ball shows the expected opposite trend. In addition, the ultrasonic measurement of wear state was verified by oil debris monitoring using on-line visual ferrography.