Relationship between an optical transparency and a filler loading in cross-linked rubber is reviewed using our results of hydrophilic and hydrophobic silica-filled peroxide cross-linked natural rubbers (NRs). The loading of hydrophilic silica shows an optical anomaly, in which the optical transparency initially decreases and then increases with increasing an amount of the loading. The optical transparencies of the hydrophobic silica filled samples, on the other hand, are almost same without depending on the amounts. The difference is ascribed to characteristics of silica networks in these samples, which are revealed by three-dimensional transmission electron microscopy. With an increase of isolated filler chains in the rubber matrix, the optical anomaly of hydrophilic silica-filled cross-linked NR is found to be significantly accelerated. Light scattering equation is also formulated using a product of the number density and squared length of the isolated chains to explain the differences.