Abstract: This paper describes the nonlinear behavior of a portal frame with tapered members. A nonlinear finite beam element with variable sections is developed in this study. The instability behavior of the tapered portal frame under uniform roof load is investigated. It is found that the lateral critical load of a frame with uniform roof load is significantly influenced by the axial forces of beams and prebuckling flexure of the members. The critical load is reduced substantially in comparison with the case when the frame is loaded on the top of the columns by two concentrated axial forces that are equivalent to the uniform roof load.