Surface modification by nanoimprint lithography (NIL) is applied to an adherend of the bonded composite joints. We developed a silicon mold with micro scale structures by photolithography and anisotropic etching. By forming CFRP on the mold with microstructures, the microstructures are transferred to the CFRP surface. Since this surface modification can be conducted during the curing of composites, it reduces the time required and costs involved in conventional surface preparation such as sand blasting, chemical etching etc. In this paper, the effect of microstructure size on the mode I fracture toughness was investigated. A size ratio A was defined and DCB tests about some values of A was conducted. As a result, it was confirmed that as A increased, fracture toughness became stronger.
