In order to clarify the filler effect on rubbers, rheo-optical measurements were performed for unfilled and filled samples of a styrene butadiene rubber/silica system. Refractive index matching was performed to increase transparency of samples and to reduce form birefringence of particles. The component stress of matrix rubber and the filler was successfully determined by using the strain-induced birefringence data. Our results were consistent with the composite model in micromechanics if the vitrified rubber layer is consider on the surface of fillers. On the basis of the experimental results, we proposed a phenomenological equation for large tensile properties for ideal filled rubbers.