In this work, the influence of graphitization degree of carbon microspheres (CMSs) on flame retardancy of poly(ethylene terephthalate) (PET) composites was examined. The graphitized CMSs (abbreviated as, TCMSs) were prepared through an annealing treatment, and both the CMSs/PET and TCMSs/PET composites with variable filler amounts were fabricated by the melt blending method. The results suggested that the graphitization degree of CMSs played an important role in the flame retardancy of PET. On the one hand, the TCMSs acted as effective heat shields to delay the ignition time (TTI), improve the initial thermal decomposition temperature of PET, decrease the heat absorption, and enhance the limiting oxygen index (LOI) values. On the other hand, the graphitized TCMSs decreased both the peak heat release rate (PHRR) and fire risk (FGI) through the formation of a more compact crosslinked carbon layer of graphitized carbon. With the addition of 2% TCMSs, the LOI of TCMSs/PET composites increased from 21% (neat PET) to 26.3%, the PHRR value decreased from 531.90 to 332.46 kW/m(2), and the residue amount increased from 9.31% to 19.31%. Compared to the CMSs, the dispersion of TCMSs in PET matrix and tensile strength of TCMSs/PET composites were also improved. Thus, these novel graphitized TCMSs show promise as efficient flame retardants for future applications. POLYM. ENG. SCI., 58:1399-1408, 2018. (c) 2017 Society of Plastics Engineers