The present invention relates to surface-modified cellulose nanofibres, which are used to improve fibre-matrix adhesion, resulting in biodegradable nanofibrillated cellulose (NFC)-polyvinyl alcohol (PVOH) composites, which can be used to form structures such as films, with excellent mechanical performance. Particularly,the present invention relates to a bio composite resin composition wherein the surface of cellulose nanofibres have been chemically modified, a method for producing such composite resin composition and a casting-sheet composite body.