Abstract: The unbending drill collar developed in Changqing Oilfield, with WOB transferring process of pressure-hydraulic pressure-tension-pressure given by its special structure, would not be crooked, unless the WOB is greater than the collar’s yield. The on-site tests of 5 wells showed that the stiffness of lower drill pipe assembly was increased to control deviation in the straight hole. At the same deviation, the big WOB can be applied to increase the penetration rate in the straight hole. The paper presents the principle, structure, lab and on-site testing results.