The experimental formula based on Lundberg-Palmgren theory has been used for the calculation of basic dynamic load rating of ball screws because its contact state resembles that of angular contact ball bearings. However, the compatibility between the theoretical and the actual rating life of ball screws has not been yet confirmed by life tests and probability theory. The inherent properties of ball screws are also not considered in this formula. In this paper, the authors deal with the development of ball screw life test machine and fatigue life test for ball screws with a total number of 31 test samples. Type and occurrence location of the initial failure of each sample are investigated. The fatigue life test data are analyzed with two and three parameters Weibull distribution functions. The basic dynamic load rating of the ball screws are also estimated from the life test data. The results are summarized as follows: (1) Initial failures are generated at the surface of the balls and the nut raceway as a spalling. (2) The life distribution of the test data fairy well fits three parameters Weibull distribution function. (3) Basic dynamic load rating, which is estimated by the life test, is approximately in agreement with that of manufacturer’s catalog.