A novel polymeric flame retardant (FR) with phosphorous-nitrogen pendent groups (PVP-P-DCA) was synthesized by polyvinyl alcohol (PVA), phosphoric acid, and dicyandiamide (DCA). Its structure was characterized by FTIR, 1H, 13C, 31P NMR, elemental analysis and TG. Poly(ethylene terephthalate) (PET)/cotton blends (T/C) was treated via pad-dry-cure process with PVP-P-DCA. The surface of FR-T/C was characterized by FTIR and SEM. The fabric performance was examined by whiteness and tensile property. The fire behavior was performed by limited oxygen index (LOI), vertical burning test, cone calorimetry test. The durability effect, thermal stability, and flame retardancy mechanism of the treated T/C were investigated by TGA, FTIR, and SEM. The results show that PVP-P-DCA is not only an effective FR but also a good char-forming agent for the T/C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.