Abstract: The characteristics of seismic-induced inelastic torsion in frame-bent structures in the main buildings of large thermal power plants are investigated. On the basis of a real engineering design scheme, nonlinear finite element models for seismic time-history analysis are established in SAP2000. The variation of structural lateral displacement, nonlinear torsion and the internal forces of main structural elements are analyzed. It is revealed that the buckling of the steel braces is the main reason for the nonlinear torsion and its rapid increment. The presence of nonlinear torsion contributes to a distortional displacement field, and could be harmful to the key elements of low ductility that connect the roof truss or crane hoist etc. to the structure. The ratio of torsional displacement to lateral displacement defined in some standards is studied to find that it hardly suites to the situation of inelastic torsion. This phenomenon and its reason are analyzed. Some technical suggestions to decrease the nonlinear torsion are put forward.