Abstract: The mode Ⅱ delamination of a glass fiber reinforced epoxy composite was studied by both static and fatigue tests.The ultrasound technic was used to measure the elastic moduli of studied composites.The measured results are proved to agree well with that obtained with mixed rulers.ENF specimens were used to obtain G_ⅡC and the relationship between da/dN and ΔG_Ⅱ.By theoretical analysis,the G_Ⅱ value calculated with classical beam theories is 3% lower than the exact value.The observation under SEM shows that different fracture modes can result in different microscopic aspects of rupture surface.