A novel liquid monomer (DOPO-VTS) containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and silicon was synthesized by allowing DOPO to react with vinyltrimethoxysilane (VTS). The DOPO-based organic/inorganic hybrid materials were prepared by the sol-gel and UV-curing methods. The scanning electron microscope results showed that the inorganic substance exhibited good compatibility with the epoxy acrylate (EA) matrix. The high content of organic/inorganic networks contributes an excellent flame retardancy to epoxy acrylate, which can be confirmed by the limiting oxygen index and microscale combustion calorimeter results. The thermogravimetic analysis (TGA) data indicate that the EA/DOPO-VTS hybrid materials possess higher thermal stability than that of pure EA at high temperature. From the direct pyrolysis/mass (DP-MS) analysis, the organic/inorganic networks can play the role of radical trap, which would consume highly reactive hydrogen and hydroxy radicals during combustion. Besides, the dynamic FTIR and DP-MS were used to analyze the thermal degradation mechanism of the organic/inorganic hybrids.