We studied patterned ITO layers of PDP thin films on glass substrates using spectroscopic ellipsometry. The optical property of ITO is expressed with the optical model based on two Lorentz oscillators. The effect of patterned ITO is calculated by taking the weighted average of reflectance in proportion to ITO coverage. The relative coverage of ITO is determined by using the model analysis of spectroellipsometric data. The difference of ITO coverage obtained by the best-fit model analysis of ellipsometric spectra to the expected one is critically examined and suggestions are made to minimize the observed discrepancy.