The idea of rugged polycrystalline Si plate to increase dynamic random access memory capacitor is now going to be realized in mass production, Effective surface areas of polycrystalline Si films were evaluated by measuring xenon adsorption before and after surface roughened process. The surface area was increased by 1.8 times due to roughened process. This result is consistent with the value of 2.1 obtained from capacitance measurements on the samples prepared under the same condition. This work shows the applicability of ultrahigh sensitive gas adsorption to thin film surface characterization. Gas adsorption gives a new quantitative measure for analyzing surface morphology in addition to the qualitative observations by scanning electron microscopy and atomic force microscopy.