Abstract: Reliability analysis of the ball screw pair is helpful to the improvement of transmission precision of machine tools. Based on Hertz contact theory, the contact deformation of the ball screw pair was analyzed, with the contact deformation function gotten. Without the distribution function of the contact deformation, the motion reliability of the ball screw pair was analyzed by the Monte Carlo simulation. Generating random numbers using MATLAB and sampling the random numbers at random, the kinematic error of the ball screw pair and motion reliability value were obtained, and the relationship between accuracy class of each part size and reliability value of ball screw pair was plotted. Evaluation of the motion reliability of the ball screw was given, which was related to size error.