A novel nano phosphorus and nitrogen based flame retardant, i.e. magnesium aminotris-(methylenepho-sphonate) (Mg-AMP) was synthesized and characterized by Fourier-transform infrared and X-ray diffraction spectroscopies, transmission electronic microscopy, and thermogravimetric analysis (TGA). Flame retarded rigid polyurethane (RPU) foams filled with nano Mg-AMP and expandable graphite (EG) were prepared, and a synergistic effect in flame retardancy between Mg-AMP and EG in RPU foam was observed. The RPU foam containing 12 wt% EG and 3 wt% Mg-AMP (sample RPU/12EG/3Mg-AMP) reached a VO rating in vertical UL-94 test with a limiting oxygen index of 29.5%. The peak heat release rate and total heat release of sample RPU/12EG/ 3Mg-AMP were obviously decreased by 52.4 and 42.9%, respectively compared with the foam with 15 wt% EG (sample RPU/15EG). Moreover, the compressive strength of sample RPU/12EG/3Mg-AMP was 35% higher than that of sample RPU/15EG, and the thermal conductivity of the former was lower than the one of the latter. TGA results indicated that the combination of Mg-AMP and EG clearly improved the thermal stability and yield of residual char of RPU foam.