Buckling tests for a GFRP unstiffened panel and stiffened panels with a top-hat stiffener are performed, and the fundamental buckling/fracture behavior including initiation of delamination is identified. Compressive stress in BWR layers is considered as one of the causes of the delamination, and the critical stress is derived by compressive material tests. To obtain a criterion for delamination produced in the curved part of the top-hat stiffener, four-pointed bending tests for curved band plates are carried out. Based on the through-thickness stress produced in the curved plate, the critical bending moment is derived. Through the validation of the critical values, the possibility to predict the initiation of the delamination is indicated.