Penta-2,3-dibromopropoxychlorocyclotriphosphazene, N3P3Cl(OCH2CHBrCH2Br)5, 〔III〕, contains phosphorus, nitrogen and bromine which are reported to react with cellulose to give durable flame retardant properties. This model compound III was synthesized by the following methods and identified. Hexachlorocyclotriphosphazene and 2, 3-dibromopropanol was refluxed in benzene solvent catalysed by pyridine for 60 hrs and purified by vacuum distillation and by liquid chromatography. This compound III was identified by IR, mass, GC and elemental analysis of C,H,N and halogens. This compound III reacts with cotton through a condensation reaction to make a durable chemical bonding by interaction of P-Cl bond and OH group of cellulose. This was identified by study of IR and modified combustion method and by comparing its thermal behavior by TGA and DSC analysis with that of untreated cotton. Finished cotton fabric showed a notable flame retardant properties as limited oxygen index was 0.275 in contrast with 0.175 for untreated cotton fabric.