A flame retardant system containing 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) and ammonium sulfamate (AMS) was proven to be effective on improving the fire performance of polyethylene terephthalate (PET) fabric The flammability tests indicate that this FR system can impart dripping resistance to PET fabric and the limiting oxygen index value can reach up to 28. The thermal behavior of PET fabric was investigated by thermo-gravimetric analysis. The results indicate that the residual char of treated samples was significantly improved by the presence of HEDP and AMS.,The morphology of treated PET fabric residue was also characterized by scanning electron microscope analysis It has been suggested that intumescent char structures have been formed.