Abstract: The unstable vibration fault phenomenon occurring on the steam turbine of a third generation nuclear power in China during the commissioning process was studied. The vibration symptom including trend, spectrum, shaft orbit and the vibration during speed up and speed down process were presented. The turbine structure, unit maintenance data and operation mode were given. Using the Ishikawa method and a process of fault elimination, the fault was thought to be rotor to stator rub. Considering the operational temperature change at seal section and the clearance value of blade tip and shaft seal, it is considered that the rub occurs at the shaft seal. From the measured data, the symptom of rub fault is vibration instability, especially the instability vibration of the synchronous component. The rub phenomenon at different states is different. It is explained from the perspective of the balance of rub induced input heat and the heat output from shaft using rotor dynamic theory. In the early fault stage, the rub induced input heat is less than the output heat, and the vibration is stable. If the input heat is approximately equal to the output heat, the vibration is in a critical stability state. Vibration turns to be unstable. In the later stage of rub fault, the input heat is larger than the output heat. There is a temperature difference in the shaft section. In this stage, the section temperature difference increases gradually. The shaft turns to be thermally bent and creates an unbalanced force. The vibration begins to diverge. If the unit is shut down, the vibration is much greater than that during startup. The degree of rub can be judged according to the amplitude difference during startup and shutdown, the maximum vibration during shutdown and rotating shaft eccentricity under turning state. On the premise of closely monitoring the vibration, the shaft sealing clearance can be gradually increased through the rub. With the multiple startup and shutdown, the rub fault can be eliminated automatically. The sealing clearance reaches the optimum value. The method is effective if the unit has no chance of maintenance. During unit maintenance, the shaft sealing clearance can be adjusted according to the observed rub marks. In the run up process, the journal will move in the direction of rotation. The sealing clearance on the side in the direction of rotation should be greater than that on the other side during installation. The adjustment of sealing clearance also needs to consider the temperature difference between the upper and lower cylinders of the cylinder during different operation state.