Abstract: The mode II interlaminar fracture toughness (G_IIC) of composites interleaved with a series of boron nitride nanoplate (BNNP) or graphene oxide (GO) modified phenolphthalein modified polyaryletherketone (PEK-C) films and BNNP modified blend films PEK-C and polyphenylphosphonic acid diphenyl sulfone ester (PSPPP) were studied. The results show that huge improvements of the G_IIC was found for the composites interleaved with nanoplate modified PEK-C films. Compared to the control, the G_IIC of the composite interleaved with a neat PEK-C film increased by 115%, while these of the composites interleaved with the corresponding 1wt% BNNP, 1wt% GO and 5wt% BNNP modified films increased by 367%, 344% and 278%, respectively. The G_IIC of composites interleaved with the PEKC-PSPPP blend film and the corresponding 1wt% BNNP modified film increased by 21.8% and 101.4%, respectively. The microstructure analysis and mechanism study show that the synergistic toughening effect of nanoplates and the toughening resin includes: (1) The introduction of nanoplates changed the dissolution behavior of the interleaves in the matrix resin and better maintained the interlayer toughening structures. The cracks kept propagating at the interfaces or inside the toughening interleaves. (2) The bridging effect of nanoplates enhanced the interfacial adhesion between toughening interlayer structures and carbon plies. (3) The presence of nanoplates induced the crack deflection and improved the roughness of the fracture surfaces. The above synergistic toughening mechanisms, especially the novel mechanism (1), can provide useful references for the development of new toughening materials and structures for the toughening of composites.