In order to discuss the health monitoring method using time-frequency analysis on the vibration of gear sets, the fatigue test employed for a steel gear pair was carried out using a power circulating gear testing machine. Furthermore, the dynamic characteristics of both steel and sintered gears were measured using a power circulating gear testing machine and were analyzed in a time-frequency domain by the continuous and discrete wavelet transforms. In the case of the failure at the dedendum surface of driven gear, not only the vibration of the gear sets but also the condition of tooth meshing would have to be taken into consideration to diagnose the failed teeth