A phosphorus-nitrogen containing compound, N-(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-N-hexylformamide-2-propenyl acid (DPHPA), was synthesized and characterized. A novel flame retardant, namely DPHPA modified layered double hydroxides (DPHPA-LDHs), was prepared by ion-exchange of LDHs with DPHPA. The results from FT-IR. XPS, XRD and TEM showed that DPHPA intercalated LDHs and PMMA-DPHPA-LDHs nanocomposites had been prepared by in situ polymerization of PMMA with flame retardant DPHPA-LDHs. The storage modulus (E') and glass transition temperature (T(g)) of the nanocomposites were enhanced by incorporating DPHPA-LDHs into the PMMA. The results of flammability showed that the addition of flame retardant DPHPA-LDHs enhanced the flame retardancy of PMMA significantly. Compared to pure PMMA, the PHRR of the nanocomposites filled with 5 wt% DPHPA-LDHs is reduced by about 35%. The results of SEM and TEM showed that a compact and dense intumescent char is formed for PMMA-DPHPA-LDHs nanocomposites after combustion. The functionalization of LDHs by intumescent flame retardant DPHPA can improve both the dispersion of LDHs in the polymer matrix and flame retardancy of the nanocomposites. (C) 2011 Elsevier B.V. All rights reserved.