The thermal decomposition of poly[diethyl 2-(methacryloyloxy)ethyl phosphate] (PDMP) by free-radical polymerization was studied by the combination of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) with thermogravimetric analysis/mass spectrometry (TG/MS), in which the former identified directly the compositions of the evolved gases and the latter could further obtain real-time evolution curves of pyrolysates. A decomposition of PDMP was a two-stage process: The dominant mechanism of the first stage was initialized by hydrolysis at the phosphate ester linkage to evolve hydroxyethyl methacrylate (HEMA), triethylphosphate, and ethanol as major products; then, subsequently, the dominant mechanism of the second stage was a main-chain random scission to form acetaldehyde and water as major products along with carbonization of about 24% char yield of a solid residue at 600degreesC. Therefore, the reaction of phosphate groups into the backbone of polymeric materials could provide better flame retardancy.