A new surface modification technique for PET films is proposed. This technique, called VPI modification technique, is a combination of two processes: The first step involves the deposition of vinylphthalimide (VPI) on the PET film surfaces, followed by Ar plasma irradiation of the VPI-covered film surfaces. The VPI modification technique led to large increases in the N/C atom ratio on the PET film surfaces. On the VPI-modified PET film surface, a new N-1s peak containing two components due to amide groups as well as imide groups appeared. The C-1s signal for the VPI-modified PET film surface also showed a new component due to ketone groups. These changes indicate that VPI reacted with the PET film surfaces to form nitrogen-containing groups. VPI modification made PET film surfaces hydrophilic. The VPI-modified film surfaces showed a decrease in water contact angle from 73 degrees to 48-56 degrees.