Abstract: Effects of subcritical quenching on the microstructure and the mechanical properties of S135 drill pipe steel were studied by means of optical microscopy (OM), scanning electron microscopy (SEM) and mechanical property testing. The results show that the ferrite-martensite dual phase microstructure of S135 drill pipe steel were obtained in the range of 760-800 ℃. With increasing of subcritical quenching temperature, the volume fraction of martensite, the strength and hardness of the steel increased, and the volume fraction of ferrite, the plasticity and toughness of the steel decreased, the fracture mode changed from ductile fracture to brittle fracture, the fracture morphology changed from dimples to cleavage.