Abstract: Melamine methylphosphonate (MMP) was synthesized through a reaction of methylphosphonic acid with melamine, and its chemical structure was characterized by Fourier transform infrared spectroscopy.Thermogravimetric analysis (TG) indicated that MMP exhibited an initial thermal decomposition temperature of 228 ℃ and a char yield of 41.2 wt.% at 800 ℃, suggesting an excellent thermal stability and charring capability. MMP was used for flame retardancy of unsaturated polyester resin (UPR), and the resulting thermosetting compounds achieved a UL 94 V-0 classification in the vertical burning experiment and a limiting oxygen index of 38.5 vol.% when 21 wt.% MMP was incorporated. This result demonstrated that the synthesized MMP had a high efficient flame retarding effect on UPR. The TG results indicated the introduction of MMP could promote the decomposition and charring of UPR and thus improved its char formation and thermal stability at high temperature. Scanning electron microscopic observation confirmed the formation of a compact and expanded char layer for the compounds during combustion. It was believed that the formed char layer inhibited the mass and heat transfer and consequently protected the underlying materials from further degradation. As a result, the flame retardant performance of UPR compounds was enhanced.