Nanoindentation has been applied to characterize the ultraviolet hardened layers of acryl resin coatings deposited on a polyethylene terephthalate (PET) film. The thicknesses of acryl resin and PET are 5μm and 50μm, respecetively. The hardness and elastic modulus of the hardened acryl layers were systematically examined as a function of indenter penetration depth using a standard nanoindentation method with a Berkovich diamond indenter. A significant decrease in hardness and an increase in elastic modulus have been observed at a penetration depth of less than 100 nm. In addition, the hardness in the vicinity of surface gradually decreases with the oxygen contents. It is considered that such softening in the surface layer is caused by a radical polymerization reduction due to the reaction of the radical with oxygen.