Effects of shaft shape errors are studied on dynamic characteristics of a rotor-bearing system. Stability characteristics of the cylindrical journal bearing are studied. It is shown that the rotating speed at which the oil whip occurs increases when the shape errors exit. And, there is a threshold speed of the bearing with shaft shape errors; before the speed is increased to the threshold, orbits of the center of the journal decrease, and when the speed exceeds the threshold, the orbits increase dramatically and oil whip appears. Furthermore, the quantitative relationship between shaft shape errors and bearing reaction forces of the rotor-bearing system is obtained, which is verified by experiments using rotors with different machining precisions. In order to reduce computing time, variational principle is applied when solving Reynolds’ equation.