Abstract: Planetary gearboxes have some advantages, including big transmission ratio and high transmission efficiency. Because its structures are more complex than that of fixed-axis gears, the faults such as petting and fatigue crack occur easily under varying conditions. In order to diagnose planetary gearbox faults effectively, a new method based on torsional vibration signals is presented in this paper. Firstly, the torsional vibration signals are collected by an incremental encoder. Compared with the features of transverse vibration signals, the facts show that the torsional vibration signals has simpler frequency spectrum, resulting in sensitizing to the planetary gearbox damage. Via analyzing an experimental torsional vibration signal, the amplitude of fault feature with failure is almost two times as high as normal. The results show that the proposed method which collects torsional vibration signals in form of angular velocity can detect the planetary gearbox faults and provide an effective way to diagnose the planetary gearbox faults.