Abstract: This paper investigates the buckling of thin-walled cantilevers. Two different theories available in the literature are discussed, and a new one is presented which takes into account the nonlinear shear strain energy and the nonlinear transverse normal strain energy. The latter strain energy term has been ignored by previous researchers Comparison is made in this paper between the results of the present theory and those of previous experimental results for cantilevers with monosymmetrical sections. It is found that the results of the present theory is in a good agreement with experimental ones, and great discrepances are found between the theory neglecting the nonlinear transverse normal strain energy and the experimental results. Cantilevers with doubly symmetrical sections under uniform moment, uniformly transverse distributed load and concentrated load at free end are analyzed, respectively. Accurate and simple formulae for predicting the critical moments are proposed in which the effects of the load acting position are included.