Abstract: By considering the geometric and material nonlinearity, residual stress, and initial geometric imperfection, I-section models are analyzed under bending and compression. The moment-curvature curves are obtained. The relation between ductility factors and slenderness of I-section are obtained and approximate formulas for the ductility factors are proposed. According to the classification of steel cross-sections oriented to the aseismic design, the paper tentatively proposed the limiting values of slenderness of the I-section. The limiting values is obtained by using the relationship between the system ductility and the cross-sectional ductility, and based on whether overstrength factor is included in the structural performance factor. The limiting values of slenderness are expressed by the correlation of width-to-thickness ratio of flange and web and the correlation formula is obtained.