Factors affecting photografting (λ > 300 nm) of methacrylic acid on low-density polyethylene film were investigated in liquid-phase system with water. Benzophenone was used as a sensitizer by coating it on the film surface. Factors examined were monomer concentration (1.3 wt% to 10.0 wt%), polymerization temperature (30°C to 70°C), and film thickness (30 μm and 80 μm). It was found that grafted polymer is formed preferentially as compared with homopolymer under conditions such as monomer concentration higher than 6.0 wt%, polymerization temperature higher than 50°C, and film thickness of 30 μm. The structure of the grafted samples obtained in the above systems was characterized by the grafted chains distributing over the film and the flat appearance of film surface. In the grafting systems using the monomer concentration lower than 6.0 wt%, the polymerization temperature lower than 50°C, and the film thickness of 80 μm, homopolymer was formed predominantly. The resultant grafted chains localized mainly on the film surface, which appeared to be grainy.