Abstract: A nonlinear strain energy for flexural-torsional buckling of web-tapered thin-walled beams is derived. Based on the derived total strain energy, finite element (FE) analysis is carried out for cantilevered beams and simply supported beams under concentrated loads and uniformly distributed loads. The results are compared with ANSYS FE analysis based on shell elements. The presented theory predicts good results compared with currently available other theories. By using the proposed theory and FE analysis, critical moments of simply supported beams with linearly tapered sections under linearly varying bending moments are obtained, then, an approximate formula with good accuracy is proposed, which has a similar expression to that for beams of uniform cross-section.