Abstract: Aiming at the problem that the circumferential and axial residual stresses on machined workpiece surface were failed to reflect the value and direction of extreme residual stress, cutting experiments were carried out on 300M steel after hard turning, and residual principle stress values and direction angles of extreme residual stress were calculated by combining the plane stress status analysis with the residual stresses measured by using X-ray diffraction on circumferential, axial and 45° directions. The effects of cutting parameters on residual principal stress were studied. The results show that the residual principal stress tended to increase as cutting speed increased, to decrease as cutting depth increased, and to change into a serrated pattern as feed rate increased. The direction angle of extreme residual stress changed little as cutting speed increased, first decreased and then kept steady as feed rate increased, and decreased as back cutting depth increased.