In this paper, a new exact formulation for calculating the elastic buckling load of semi-rigid steel frames withtapered columns will be obtained. The presented methodology is based on the precise solution of the governingdifferential equations for buckling of the uniform and non-uniform frames. Then, the effects of the non-prismaticmembers, with a parabolic stiffness variation, flexibility of connections, and lateral bracing on the buckling lengthfactor and corresponding critical load of a portal frame will be investigated parametrically. Results show thecombined effect of aforenamed parameter on the critical buckling load and corresponding equivalent bucklinglength factor of portal steel frames is very significant.