A washing machine having a ball balancer to reduce vibration generated during dehydrating process and a vibration controlling method thereof are disclosed. In a washing machine having a ball balancer, a dehydrating process starts while residual water having a preset water level remains in the tub without draining water completely when entering the dehydrating process, and the quantity of residual water is set differently according to eccentricity so that excessive vibrations of a tub may be reduced using the weight of the residual water while passing through a resonant point. Speed of a motor is maintained at a preset speed lower than the resonant point for a preset time to release balls in the ball balancer so that excessive vibrations of the tub generated by concentrated balls and unbalanced laundry may be surely reduced. After passing through the resonant point, the residual water is drained to reduce drain noise and to prevent delay of dehydrating time.