Abstract: The running-in and wear tests of a piston ring against a cylinder available from marine diesel engine are conducted on a pin-on-disc tester. The friction vibration signals from the running-in process are collected. The nyquist stability criterion is used for analyzing the stability characteristics of the running-in system through the nyquist curve of the friction vibration. It is shown that the stability of the running-in system can be characterized by he nyquist curve of the friction vibration. When the friction pairs start running (0~10 min), the point (-1, j0) is not surrounded by the nyquist curve of the friction vibration, the running-in system runs under the stable wear state. When the friction pairs run during the running-in and wear process (10~40 min) , the point (-1, j0) is surrounded by the nyquist curve of the friction vibration, the running-in system runs under the unstable wear state. When the friction pairs achieve the running-in (after 40 min), the point (-1, j0) is not surrounded by the nyquist curve of the friction vibration, the running-in system runs under the stable wear state. Therefore the stability characteristics of running-in process can be revealed by the running-in system properties.