Abstract: A phosphorus and silicon containing liquid monomer (9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide-vinyltrimethoxysilane (DOPO-VTS)) was synthesized by the reaction between DOPO and VTS. DOPO-VTS and methacryloxypropyltrimethoxylsilane were introduced into unsaturated polyester resin to prepare flame retardant UPR/SiO2 (FR-UPR/SiO2) hybrid materials by sol-gel method and curing process. DOPO-VTS contributes excellent flame retardancy to UPR matrix, which was confirmed by the limiting oxygen index and microscale combustion calorimeter results. The thermogravimetric analysis (TGA) results indicate that the FR-UPR/SiO2 hybrid materials possess higher thermal stability and residual char yields than those of pure UPR at high temperature region. The thermal degradation of materials was investigated by TGA/infrared spectrometry (TG-IR) and real-time infrared spectrometry (RT-IR), providing insight into the thermal degradation mechanism. Moreover, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to explore the morphologies and chemical components of the residual char. Copyright (c) 2013 John Wiley &, Sons, Ltd.