Methacrylic acid was graft polymerized with benzoyl peroxide in itiator onto the commercial PET film. Graft polymerization was carried out in emulsion, which consisted of BPO (1), chlorobenzene(8) as swelling agent, tween 80(1) and sodium lauryl sulfate(1) as emulsifier and water(1,000). Original PET film has poor reactivities and, so the film preswdled with benzyl alcohol($150^{circ}C$, 1hr) was also examined. The graft yield of PET film was increased with reaction temperature and monomer concentration. The graft yield of preswelled PET film was higher than that of no-treated PET film. Moisture regain was linearly increased with graft yield. DSC thermodiagram showed the Tm of grafted PET film was same as that of original PET film. Grafted PET film was dyed with methylene blue solution. The photograph of the cross section showed that grafting was occured not at the center but near surfaces.