Buckling modes of pultruded Fiber Reinforced Polymer (FRP) beams are analyzed in this paper. The study is performed on the basis of two mechanical models recently proposed by the authors with regard to global and local buckling of composite thin-walled beams. These models are developed within the theory of small strains and moderate rotations and they take into account the contribution of shear deformation. The constitutive law here adopted is based on the homogenization of the material properties at the macro scale level. With regard to local stability, the junctions are considered as semi-rigid connections, whose stiffness is strongly influenced by the microstructure of the material. A discussion on the effects of the beam geometry and on the failure modes is presented. They may consist in local or global buckling as well as in material failure. Further, the global buckling may be torsional or lateral. The discussion is supported by non-dimensional diagrams which can be useful in design involving “T” and “C” sections subject to axial and bending loads.