Interlaminar fracture toughness was investigated in CFRP laminates with ionomer interleaf of the thickness of 200μm. Ethylene based ionomer resin used in this study has high bonding strength with base composite. EPMA analysis revealed that the tough interphase layers where ionomer was mixed with epoxy resin were formed at the interfaces between ionomer layer and prepreg layer. Mode I fracture toughness tests were carried out using double cantilever beam specimens. The fracture toughness of ionomer interleaved CFRP was much higher than that of base CFRP. This high level of the toughness was kept constant with the increase of the crack length. Microscopic observation showed that the crack path was at the ionomer/tough interphase interface or inside the interphase region. Thus, the large plastic deformation of the ionomer resin mainly affected the great increase of the fracture toughness. Furthermore, bridged ionomer layers also affected the increase of the fracture toughness.