This paper deals with a diagnosis of tooth surface failure by the wavelet transform. The vibration acceleration of the gear box and the tooth root strain were measured and the tooth surfaces were observed during the fatigue test. A tooth surface of the gear was damaged by spalling failure with the progress of the fatigue test. Using the analysis of the vibration, a failed tooth could be diagnosed, when one tooth failed. However, in the case of adjoining plural failed teeth, it was difficult to diagnose each failed tooth. The failure positions on a tooth surface could possibly be detected using the time-frequency analysis of the dynamic tooth root strain.