Aluminum alloy extruded channel sections are often used as rafters for light roofs, columns in framed buildings and chords of roof trusses. In this paper, a numerical study is carried out on the response of aluminum alloy channel sections under minor axis bending. Numerical models were developed and validated based on the experimental data of aluminum alloy channel sections under minor-axis bending. Subsequently, parametric studies covering various cross-sectional geometries were performed to expand the numerical data. The results were used to assess the applicability of Eurocode 9 slenderness limits, revealing that the Class 2 and Class 3 limits for outstand flanges under stress gradient with peak compression at tip are too conservative. Moreover, Eurocode 9 underestimates the predicted bending strengths of slender cross-sections due to the lack of consideration of the effect of stress gradient. The equivalent stress gradient factor applied in the effective thickness approach was proposed based on the buckling coefficient of BS 5950. For channel sections bent about minor-axis with the web in compression, flanges exhibit pronounced inelastic behavior even when the web locally buckles. An alternative design method based on the ultimate shape factor of a rectangular plate is proposed for semi-compact and slender channel sections in minor-axis bending (producing compression in the web), which leads to more favorable and less scattered capacity predictions.