ORMOSIL doped with Thionine dye was prepared by sol gel method and its optical nonlinearity was measured using a single beam (Z-scan). The nonlinear refractive index n2 is about 4×10 -7 esu. An analysis shows that the nonlinear refraction of Thionine dye doped ORMOSIL is attributed to the delocalized π electron effects of organic dye.