As digital cameras have become a popular consumer product, higher standards for ink jet paper is required in order to produce better picture quality. The surface of ink jet paper is generally coated with ink-absorbing material that mainly consists of inorganic particles such as silica, alumina or hydrophilic swelling resin. The required function of the coat layer is to quickly absorb only the solvent of the ink which is distributed from the printer and to keep the dye on the surface, which is the key to determine the quality of printed pictures. In this presentation, we report the results of our analysis of the ink-absorption mechanism of the coat layer materials containing various types of silica using a mercury porosimeter. We found that the precipitated silica “Finesil X Series” keeps more dye on the surface than other types of silica because it has more pores with diameter around 100 Å, which is the suitable size for adsorbing dye. As the result, ink jet paper coated with “Finesil X Series” generates high optical densities.