A novel flame retardant composed of montmorillonite (MMT) and hexachlorocyclotriphosphazene (HCCP) was prepared in this study. MMT was chemically bonded with HCCP via a 3-aminopropyltriethoxysilane (KH-550) coupling agent. The structure was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and NMR. The preliminary application for improving the flame retardancy of poly(ethylene terephthalate) (PET) was investigated by means of the limited oxygen index, the vertical burning test, and thermogravimetric analysis. Scanning electron microscopy was used to investigate the morphology of the char residue. The results show that the novel flame retardant had excellent fire retardancy; that is, the sample achieved an increased UL-94 V-0 rating and limited oxygen index value of 31.5. The residue of the flame-retarded PET increased to 14.7% compared to the 6.2% value of pristine PET. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39625.