Conformational study of silicon backbone of poly(methylphenylsilane) functionalized with pendant azobenzene groups has been carried out. The conformation of the silicon backbone depends on the molar ratio of the Si units with and without the pendant azobenzene group, and the inflection point for the conformational change from the slowly winding helix to random coil has been found to be ca. 0.5 of the molar ratio. The conformational change has been induced by the photoisomerization of pendant azobenzene groups in the film state.