This paper proposes an analysis method for the fluctuation of the positional deviation in a feed drive system. In this analysis, the positional-deviation fluctuation due to posit ion-dependent disturbances is separated from the total deviation. The cause of the disturbance is roughly determined. The positional deviation of an actual feed drive system with a ball screw drive was analyzed by the proposed method. The positional deviation and motor torque were measured at several constant feed rates. The measured data were analyzed in the wavelength domain to distinguish the positional-deviation fluctuation due to the posit ion-dependent disturbance, in the positional deviation, seven periodic fluctuations were determined to be the position-dependent fluctuation. These fluctuations were caused by the circulation of balls in the ball screw, the run-out of the ball screw, the geometrical inaccuracy of the ball screw and the disturbance in the control system, including the servo motor. The fluctuation magnitude of the positional deviation due to the ball screw was almost 1/3 of the total magnitude. The magnitude of the positional-deviation fluctuation depended on the feed rate and the wavelength of the disturbance because of the frequency response of the feed drive system to the disturbance.