Abstract: Taking ball screw with outer cycling device as the research object,by means of mechanics analysis,a calculation model,which can calculate the contact stress of every ball and the fatigue life of ball screw with considering dimension errors,is put forward.With a concrete example,the influences of the dimension error on the contact stress and fatigue life of two situations,the first one is only one ball has dimension error and the second one is that every ball has random dimension error,are studied.The maximum contact stress of balls is mapped,and the fatigue life of ball screw is calculated.Meanwhile the influences of machining accuracy on the ball screw's fatigue life have been analyzed.The fatigue life of ball screw increases as the machining accuracy increases.