There are low frequency horizontal vibration problems in varying degree on the stator frame for operating giant hydro generator unit, those who adopted flexible support. To determine the cause of excessive vibration, the horizontal vibration of the stator frame and core were measured under the operating conditions of variable rotate speed, variable load and variable excitation. The results showed that the low frequency vibration had similar vibration characteristics in each generator unit, that is, the vibration of the stator increased as the excitation increasing and it did not matter to the change of rotate speed and load. This low frequency vibration was obviously caused by the unbalance magnetic pull. Based on the uneven of air gap, which induced by uneven magnetic pole morphology and relative eccentricity of the stator-rotor, low frequency vibration was explained.