Abstract: Based on the mechanics of materials and the elastic-plastic theory, the various bending load-deflection curves are studied on different steel grade, heat treatments and conditions in industry. The results show that the second stage of the bending load-deflection curves of steels can be simply treated as an linear relationship in the actual straightening deformation range and the straightening theorem of steel fr = K·(f - fm ) is deduced, which accurately describes the relationship between the residual plastic bending deflection and the total bending deflection after bending. When straightening according to the straightening theorem, the straightening process can be controlled easily and directly, and the straightening quality and efficiency are raised. Particularly, the efficiency of press straightening can be increased by 30 ~ 80 times. This straightening theorem fills the gap that there was no direct calculating formula of residual plastic bending deflection after bending before in the straightening field and enriches the current straightening theory and it has important scientific significance to promote the development of straightening technology and the design and manufacture of straightening equipment.