Abstract: The friction and wear tests of a piston ring against a cylinder from marine diesel engine were conducted on a pin-on-disc tester. The friction vibration signals under different wear condition were studied making use of the hamonic wavelet packet transform. It was showed that the weak friction vibration signal can be extracted using the hamonic wavelet packet transform. When the wear mechanism is slightly adhesion wear, the side-frequencies dominate the frequency spectrum of friction vibration signals, the natural frequencies don't appear in the frequency spectrum and the average amplitude of the signals is small. When the wear mechanism is abrasive wear, the natural frequencies appear in the frequency spectrum of friction vibration signals, but a large number of side-frequencies still exist and the average amplitude of the signals increases. When the wear mechanism is fatigue wear, the natural frequencies dominate the frequency spectrum of friction vibration signals, the side-frequencies significantly reduce and the average amplitude of the signals is big. Therefore The characteristic extraction of that the weak friction vibration signal can be carried out by the hamonic wavelet packet transform.