Natural rubber grafted with poly(dimethyl(methacryloyloxymethyl)phosphonate) (i.e., NR-g-PDMMMP) was prepared in latex medium via photopolymerization. Thermal and flame resistance properties of the NR-g-PDMMMP prepared with various levels of grafted PDMMMP or grafting rate (GR) were investigated. Thermal behaviors were investigated by thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). It was found that the graft copolymer exhibited phase separation with double T-g values. A shift of T(g)s toward each other was observed with increasing GR, which indicated tendency to become a single phase material. Increasing GRs also caused increasing heat and flame resistance with increasing degradation temperature and level of char residue. Furthermore, increasing level of limited oxygen index (LOI) and decreasing burning rate were observed with increasing the GR. This is attributed to increasing content of char residue of the phosphorus compound, which acted as the thermal insulation and a barrier of oxygen to transfer to the burning materials. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 255-262, 2010