Over-vibration occurred during startup of a hydro-generator unit because of permanent bending of rotor caused by operating failure. The fault diagnosis was ca rried out by studying the vibration spectrum under different operating condition s in cluding speed up, excitation, load up and full load. Result shows that permanent bending and thermal bending after excitation are the main reasons leaded to the high vibration amplitude of the generator. Relevant approach of onsite ba l ancing after straightening rotor is conducted to keep the shaft vibration at the safety level. By analyzing the vibration characteristics of a bending rotor, the ba lance response and thermal bending changes are summarized.