The effects of joint clearance on robot manipulators positional error were investigated. The kinematic model was built by using the D-H method to research the six degrees of freedom mechanical arm. The central error propagation model was established. The simulations of error distribution were conducted in MATLAB with Monee Carlo method. The distribution characteristic and probability distribution of positional error of mechanical arm's end, which is caused by the joint clearance, were obtained from the simulations at different levels of joint clearance. This research provides basis for error compensation and positional error evaluation.