Method for the identification in operation of the drive train main vibration frequency of a wind turbine, comprising online steps of: a) obtaining a input signal (r) of the generator speed (Ω); b) filtering the generator speed input signal (r) for obtaining a generator speed signal (r1) in a suitable band for representing the oscillating signal (o) comprised in the input signal (r) of the generator speed (Ω); c) extracting the drive train main vibration frequency (f) from said filtered signal (r1). The invention also comprise a method of damping the drive train vibrations of a wind turbine comprising steps of setting the generator torque reference (Trref) as a function of the generator speed (Ω) and of the drive train main vibration frequency (f) identified by the above mentioned method and wind turbine control system implementing said damping method.