Platinum nano particles dispersed tungsten trioxide (Pt/WO3) is a promising hydrogen gas sensor material which can detect it by the change of color. Hydrogen gas sensing properties of Pt/WO3 thin film strongly depend on its film structure. Particularly, the porosity and the particle size of WO3 are very important for obtaining good hydrogen sensitivity. The morphology of Pt/WO3 thin film prepared by solgel process can be modified by the heat-treatment temperature and the Ti-doping to WO3. Pt/WO3 thin films consisting of small particles and porous structure were obtained by Ti-doping. Especially, Ti-doping was very effective to obtain the small WO3 particle in spite of a high temperature heat treatment. It was considered that Ti-doping produced oxygen vacancy in WO3 lattice and this disturbed the densification of the film and the grain growth of WO3. This sample showed a good response to hydrogen gas exposure, compared with that of non-doped sample. Therefore, it was found that the surface modification by Ti-doping can enhance the optical hydrogen gas sensing property of Pt/WO3.