A beam model for non-linear buckling analysis of thin-walled functionally graded open section beam-type structures is presented. The finite element incremental equilibrium equations have been developed by UL formulation using the non-linear displacement cross-section field that accounts for large rotation effects. Three variants of beam wall are investigated: Type A – FG wall, Type B – sandwich wall with two FG skins and homogeneous ceramic core and Type C - sandwich wall with homogenous ceramic skin and FG core. Also, three types of cross sections are considered: symmetric and mono-symmetric I- and channel section. For various boundary conditions, the influence of power law index magnitude on the critical buckling loads and post-buckling responses are observed as well as the effects of skin-core-skin thickness ratios. Variation of the centre of gravity and shear centre positions depending on material distribution is also taken into account. The efficiency of the proposed algorithm has been tested considering some benchmark examples.