A large twist rotation model for the nonlinear inelastic analysis of the nonuniform torsion of I-section beams is presented in this paper. A finite-element procedure has been developed based on the model. The nonlinear equilibrium equations of beams in nonuniform torsion including the effect of the large twist rotation have been derived. The elastic-plastic behavior of beams in nonuniform torsion is studied using the finite-element procedure and the results are compared with tests. It is found that I-section beams have much larger torsional capacities than can be predicted by linear plastic collapse analysis, and that torsional failure occurs not by the formation of a mechanism but by the tensile rupture of the flanges. A method is proposed for calculating the full plastic nonuniform torque for practical design purposes.