A method for preparing photo-crosslinkable cellulose nanofibrils (CNF) was investigated. Benzophenone (BP), a UV-radical crosslinker, was chemically grafted to TEMPO-oxidized wood fibers in water/DMSO medium. This resulted in a reduction of carboxyl group content together with an increase of the number density of amide linkages. The BP-functionalized fibres were then microfluidized into TEMPO-oxidized CNF (TOCNF). As evidence of the crosslinking performance, the films of BP-activated TOCNF displayed improved water resistance upon UV curing (even under high energy sonication). In addition to improving wet strength the method is suitable for further modification of CNF, either by utilizing the remaining free carboxyl groups or via photochemical grafting of other substances onto the CNF structure.