Penta (2, 3-dibromopropoxy) allyloxycyclotriphosphazene, N3P3OCH2CH=CH2(OCH2CHBrCH2Br)5 was synthesized and identified by IR, mass spectroscopy and elemental analysis of C,H,N and halogen. This vinyl monomer could be grafted on cellulose by CeIV initiation without opening its aromatic phosphazene ring. After extraction of homopolymer, thermal behavior of grafted cellulose was studied and compared with that of pure cellulose by TGA and DSC analysis. Grafted cellulose showed two typical double peaks at 240 and 274$^{circ}C$ in DSC curve, lowered the decomposition temperature, and increased the amount of char. It suggested that decomposition mechanism of grafted cellulose is different from that of pure cellulose, which leads to improvement in flame retardant property.