A novel monomer, poly(ethylene glycol) monoacrylate aminopropyltriethoxysilane phenyl phosphate (SNP), containing phosphorus, nitrogen, and silicon was synthesized and then incorporated into a poly(methyl methacrylate) (PMMA) matrix through copolymerization and the sot-gel method to produce organic-inorganic hybrids. The chemical structure of SNP was characterized by FTIR, H-1 NMR, Si-29 NMR, and P-31 NMR spectroscopies. The Si-29 MAS NMR results for the hybrid materials suggested the formation of cross-linked networks in the hybrids. A morphological study showed that the inorganic particles were well distributed in the PMMA matrix. The hybrids retained a high transparency and exhibited a significant improvement in glass transition temperature, thermal stability, hardness, and flame retardancy upon the incorporation of SNP into the PMMA matrix. The network structure, homogeneous distribution, and char formation during degradation were proposed as three key reasons for the improved properties.