In this paper, hydrophilic poly(acrylic acid) (PAA) brushes were evenly grafted onto ultra-flat polydimethlysiloxane (PDMS) films by UV irradiation in a benzophenone (BP)/aceton/acrylic acid solution. The grafted films remain the elasticity of PDMS, which indicates the grafting reaction takes place at the superficial surface. But water affects the grafting depth much due to the good diffusion of monomers into the film.