Abstract: Constant amplitude fatigue crack growth tests were performed on center cracked 8mm thick specimens over a wide range of R-ratios.tn consideration of that it is important to determine the crack opening stress for the crack growth life prediction based on the crack closure concept and it is necessary to find a convenient way to determine the range of effective stress intensity factor Keff in engineering practice, this paper derived an empirical expression for the crack opening stress of the material tested in this investigation from the crack growth rate data da/dn. Consequently the crack growth behaviour over a wide range of R ratios can be expressed unitedly by the range of effective stress intensity factor Keff, tt is found that dimensionless crack opening stresses decrease as the applied maximum stress level increases.The results of fracture analysis indicated that the transition of fracture from tensile mode to shear mode during the tests was controlled mainly by Keff and the values of Sop/Smax increase as the cracks growth transi-tting, from the tensile mode to the shear mode.