Cross-sectional ovalization of thin-walled circular steel tube because of large plastic bending, also known as Brazier effect, usually occurs during the initial stage of tube's continuous rotary straightening process. The maximal cross-section flattening (ovalization) is an important technical parameter in tube's straightening process to control tube's bending deformation and prevent buckling. However, for the lack of special analytical model, it is determined in accordance with the specified charts developed by experienced operators on the basis of experimental data, thus it is inevitable that the local buckling will occur during some actual straightening operation. In the present paper, simplified normal strain component formulas are derived based on the thin shell theory. Then strain energy of thin-walled tube (per unit length) is obtained subsequently using the elastic-plastic theory. Finally a rational model for predicting the maximal section flattening of the thin-walled circular steel tube under its straightening process is presented by the principle of minimum potential energy. The simplified model is validated by experiments and numerical simulations. The results show that this model agrees well with the experiments and the numerical simulations with less than 10% error. This new model is expected to find its potential application in thin-walled steel tube straightening machine design. (C) 2016 Elsevier B.V. All rights reserved.