O 1s x-ray absorption spectroscopy and first principle calculation have been used to probe the size effect on the orbital character and hybridization of the conduction band of anatase TiO2 nanocrystals over two orders of magnitude in diameter (2-200 nm). The appearance and predominance of unoccupied delocalized states derived from the hybridization of antibonding O 2p and Ti 4s rather than 3d is observed when the nanoparticle size approaches the exciton radius. These results provide an experimental evidence of quantum size effect on unoccupied states in anatase TiO2 nanocrystals.