Abstract: The fatigue cracks propogation behavior of S135 drill pipe steel at different stress ratios was studied by combining fatigue properites test and data regression analysis methods, the expressions of fatigue cracks propogation rates were obtained in the steady propagation region and near-threshold region, and the fatigue fracture mechanism was discussed. The results show that the increase of stress ratio made fatigue crack propogation rate curves move to the left upper side, da/dN increase, and ΔKth decrease in linear. The fracture of near-threshold region was cleavage fracture, and that of steady propogation region was mainly fatigue strip propogation mechanism with cleavage fracture.