A novel phosphorus-containing poly (ethylene terephthalate) (PET) copolyester/nano-SiO(2) composite (PET-co-DDP/SiO(2)) was synthesized by in situ polycondensation of terephthalic acid (TPA), ethylene glycol (EG), [(6-oxide-6H-dibenz[c, e] [1,2] oxaphosphorin-6-yl)-methyl]-butanedioic acid (DDP), and nano-SiO(2). The morphology of PET nanocomposites was observed by using transmission electron microscope and scanning electron microscope. It was found that the SiO(2) nanoparticles were dispersed uniformly at nanoscale in the copolyesters with content 2 wt %. The thermal degradation behavior of PET nanocomposites was investigated by thermogravimetric analysis performed with air and nitrogen ambience. The activation energies of thermal degradation were determined using Kissinger and Flynn-Wall-Ozawa methods, respectively. The results obtained from Kissinger method showed that the activation energy was increased with the introduction of SiO(2). Moreover, the activation energy is decreased for PET-co-DDP system in nitrogen and air. The results also indicated that the SiO(2) and DDP had synergic effect on the early decomposition and the late charring in air. Furthermore, in the PET-co-DDP/SiO(2) system, the activation energy increased when the DDP component increased. However, the opposite results were obtained when the Flynn-Wall-Ozawa method was used. That was because the Doyle approximation stands correct as the conversion degree is from 5% to 20%. The effects of SiO(2) and DDP on the PET thermal degradation were lower in nitrogen than in air. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 1993-2003, 2011