Abstract: Effects of through-thickness stitching on Mode II interlaminar fracture toughness(G_II) and modes of delamination of composite laminates were investigated by performing end-notched flexure(ENF) tests. Vertical displacement of loading point and crack shear displacement(CSD) between upper and lower crack surfaces at the end of ENF specimens were measured. Different stitch yarns and stitch densities were considered. The addition of stitching was found to decrease the initiation interlaminar fracture toughness G_IIi, but can effectively suppress the growth of delamination.