New photosensitive monomers having multiple methacryloyloxy groups were synthesized by the reaction of naphthalene and biphenyl derivatives containing two or more glycidyloxy groups with methacrylic acid. The obtained photosensitive monomers were mixed with polymerization initiators (1 wt% AIBN and/or BIPE) and a reactive diluent (20 wt% DPCA60). The mixtures were termed as compounds. The compounds were converted into the insoluble gel products by heating or photo-irradiation with an ultra high-pressure mercury lamp. We also measured the heat resistance of the insoluble gel products by thermogravimetry and investigated the correlation between the heat resistance and the chemical structures of photosensitive methacrylate composites. All of the insoluble gel products showed the heat-resistance over 260 8C and particularly the compound having the largest number of methacryloyloxy groups had the highest heat-resistance among them. This compound exhibited the highest consumption rate of the photosensitive group and it was in accordance with the result of the reactive photo-polymerizability measured by the gray scale method. q