Polymer/nanoclay composite layer was coated on plastic substrates for flexible display to improve its barrier properties. Sodium montmorillonite (Na+-MMT) nananoclay was modified by organic intercalation agents to enhance the d-spacing and compatibility with polymer. Combination of 3-aminopropyl trimethoxysilane(APS) and 3-trimethoxysilyl propyl methacrylate(MAPTMS) achieved d-spacing as 5.78 nm at the best ratio of 1:3 as predicted in mechanism study. Modified nanoclay formed a pre-polymer mixture with polyurethane-acrylate (PUA) to make a composite layer by UV-curing process. For PEN substrate oxygen transmittance rate(OTR) was much reduced to below 0.22 cc/m(2) day, which wasmuch better than the other composite layer with common nanoclay. Water vapor transmission rate(WVTR) was also reduced after nanoclay composite layer formation on polyethylene naphthalate(PEN) and polyethersulfone(PES) substrates. Excess amount of nanoclay tended to agglomerate to deteriorate the barrier properties. Optical properties of PUA/clay composite layer were slightly affected and showed good performances for their application to display devices.