Abstract: The combinations of cerium phenylphosphonate (CeHPP) and decabromodiphenyl oxide (DBDPO) were used for flame retardancy of glass fiber reinforced poly(ethylene terephthalate)(GF/PET) composites by using melt blending. Thermograviment analysis(TGA) was employed to investigate the thermal decomposition behavior of the flame retardant DBDPO-CeHPP-GF/PET composites. The combustion properties of DBDPO-CeHPP-GF/PET composites were measured by using limiting oxygen index (LOI), underwriters laboratories 94 (UL-94) and microscale combustion calorimeter (MCC). SEM was used to investigate the microtopography of char residues of DBDPO-CeHPP-GF/PET composites. The thermal properties and flame retardance of DBDPO-CeHPP-GF/PET composites are significantly improved by the incorporation of the combinations of CeHPP and DBDPO. For DBDPO-CeHPP-GF/PET composite with mass ratio of GF/PET composites:DBDPO:CeHPP of 91:6:3, the UL-94 V-0 classification and high LOI (29.5%) are achieved. The total heat release (THR), peak heat release rate (PHRR) and heat release capacity (HRC) of DBDPO-CeHPP-GF/PET composites at this ratio are respectively reduced by 10.2%, 13.1% and 12.8% comparing with the pure GF/PET composites. And the flame retardant mechanism of DBDPO-CeHPP-GF/PET composites was analyzed based on the SEM results of char residues.