The graft density and surface topography of ultra-flat polydimethylsiloxan films grafted with poly(acrylic acid) brushes by UV irradiation are investigated. The graft density keeps increase with the irradiation time and with the monomer concentration to a maximum, after which it remains unchanged in the former case or drops in the latter. Atomic force microscopy results show that a longer irradiation time, a higher monomer concentration and the addition of ethanol in the grafting solution are favorable to obtain the grafted polydimethylsiloxan films with flat surface. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2266-2271, 2012