Abstract: This paper presents methods for estimating critical speed and damping ratio of a rotor and predicting the rotor response to imbalance at the critical speed. The estimation and prediction are carried out by means of the vibration values measured during the escalation stage of the rotor speed but below critical speed. In this way the critical speed and damping ratio can be obtained without running the rotor over its critical speeds, therefore a violent and dangerous vibration of the rotor at critical speed can be avoided. Additionally the prediction of the rotor response to imbalance at critical speed can give an assessment of condition of the rotor balance. Experimental results show that the critical speed and damping ratio can satisfactorily be estimated by these methods below 80 percent of the critical speed.