Separation at the web-flange junction is a common failure mode of pultruded glass fiber-reinforced (GFRP) beams subjected to bending. The causes of this separation appear to depend on the presence of lateral supports to prevent lateral buckling. To clarify the driving mechanisms, four-point bending experiments were carried out on pultruded GFRP girders. Lateral buckling was prevented by using lateral supports. Web-flange separation failure due to exceeding the shear strength was observed before any buckling was seen. Furthermore, nonlinear FEA was performed to identify the critical stress states of GFRP beams from the research literature, without any lateral supports in the post-buckling phase. In this case, based on numerical calculation, the critical stress states and their locations depended significantly on the shape of initial imperfections. The ultimate loads, with or without lateral supports, were predicted by a modified von Mises criterion applied to the stress states at the web-flange junction.