In order to investigate the wettability of polymer surface, the surface tension of poly(4-vinyl pyridine) (P-4-VP) and its derivative was obtained by Zisman's plot and extended Fowkes equation. The values of surface tension of modified P-4-VP were related to the structural change of the sample due to the treatment of hydrochloric acid. It was found that the values of surface tension obtained by Zisman's plot were in good accord with those calculated by extended Fowkes equation. The value of surface tension of untreated P-4-VP was high; 51-52dyn/cm, by contrast, that of the modified sample decreased to 42 to 44dyn/cm. It was notable that the molecular interaction force of polar group, among factors contributing to γs, became zero, moreover, the hydrogen bonding force decreased by the formation of pyridinium chloride. The above result was supported by the infrared spectra indicating that P-4-VP transformed to Poly(4-vinylpyridinium chloride) by the treatment with hydrochloric acid.