Several methods to enhance the barrier property of plastic substrate were tested. Inorganic layer of SiN(x)O(y) was deposited by PECVD, and barrier properties were enhanced with increased deposition time. Pre-treatment of substrate with Ar/O(2) plasma modified the surface for better deposition of layer with improved barrier properties. Organosilane-nanoclay composite layer was formed by curing the dope solution on the surface through hydrogen bonding formation, and it especially enhanced the barrier property to oxygen. Multilayer formed by combination of inorganic layer on organosilane-nanoclay composite layer attained the best barrier properties among the several methods tested. Inorganic layer and organosilane-nanoclay composite layer were much damaged by bending test to show the deterioration of barrier properties. However, multilayer still retained the barrier property even after the bending test. Multilayer deposited on PES substrate was successful in terms of optical performance for flexible plastic substrate.