In order to study the detergency phenomena in the aspects of enegetics, surface free energy of substrates were calculated. Substrates used in this study were prepared by grafting methacrylic acid (MAA) on PET film and surface free energy of PET film was controlled by grafting ratio of MAA. Surface free energy of substrate, that is surface tension, consisted of dispersion force and polar force components were estimated by means of contact angle measurement and appling these values to extended Fowkes` equation. The results showed that surface tension of original PET film were r^d=35.9dyne/cm, r^p=5.5dyne/cm and r=41.4dyne/cm and with increasing grafting ratio, the dispersion force component became small, while the polar force component became large and total surface free energy became large. Thus the surface tension of grafted PET film ranged 7^d=30.9-25.2dyne/cm, r^p=12.4-28.3dyne/cm and r=43.3-52.5dyne/cm.