Zirconium phosphate (ZrP) was prepared at various sizes and aspect ratios to be used for epoxy-ZrP nanocomposites formation for enhancement of the barrier properties. ZrP syntheses were performed at various concentrations of phosphoric acid via reflux and hydrothermal methods. The synthesized ZrP was modified using Jeffamine M1,000 to increase the interlayer distance and compatibility with polymer. Epoxy-ZrP nanocomposite layers were coated on sputtered PET substrates to improve the barrier properties. The dispersed and arranged ZrP structure was confirmed by transmission electron microscopy (TEM). The water vapor transmission rate (WVTR) was decreased after nanocomposite coating on sputtered PET substrate with increased ZrP size, which dramatically improved the barrier performance. Optimum size of ZrP was 500 nm which increased the aspect ratio to 250, and its composite coating achieved the WVTR value down to 10−4 g/m2/day from sputtered PET (10-2 g/m2/day) substrate before composite coating. The lag time was measured at over 100 h with the Ca cell test under conditions of 85℃ and 85% RH. Optical properties were little affected by an increase in the size of ZrP and were similar to those of sputtered PET substrates.