Current seismic design procedures implicitly permit the inelastic structural deformations under strong ground motions for economic reasons. The local ductility is used as a parameter for evaluating the available inelastic performances of structures. Current code provisions take into account the ductility at the level of section, but it is necessary to study member ductility. One of the main factors influencing the value of ductility is moment gradient. This research presents a new method to determine ductility for constant-moment loading of flexural members based on theoretical and numerical studies. A numerical study was carried out on welded I-beams under a monotonic static loading, using nonlinear buckling analysis considering local and overall instabilities. The results illustrate that this new method is more accurate than others, can be used to predict ductility for constant-moment loading, and may be employed for different gradients of moments.