Abstract: Buckling is a critical design consideration for pultruded glass fiber reinforced polymer (GFRP) flexural members. To investigate the failure mechanisms and evaluate existing design approaches, this study conducted systematic three-point bending tests on pultruded GFRP I-section beams. Four cross-sections, each with five spans, were designed, considering the flange width-to-thickness ratio and member slenderness ratio as key parameters. The failure processes and modes were analyzed by synthesizing test phenomena, load-displacement curves, and strain data. The results show that the failure modes can be classified into material failure, lateral-torsional buckling, and local-global interactive buckling. The governing pattern is as follows: failure is governed by material strength when the slenderness ratio is less than 58, by lateral-torsional buckling when the slenderness ratio exceeds 69 and the width-to-thickness ratio is below 8.88, and by interactive buckling when the slenderness ratio exceeds 69 and the width-to-thickness ratio is above 8.88. As the specimen slenderness ratio exceeds 58 and continues to increase, the governing failure mechanism gradually shifts from strength-controlled behavior to stability-controlled behavior. Comparisons between test results and predictions from current standards and theoretical methods reveal that the methods generally yield conservative estimates for lateral-torsional buckling capacity, with limited accuracy for interactive buckling, highlighting the need for further refinement.