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A novel phosphorus-nitrogen flame retardant, nano zirconium amino- tris -(methylenephosphonate) (Zr-AMP), successfully synthesized by allowing amino trimethylene phosphonic acid (AMP) to react with ZrOCl·8H 2 O, was incorporated into rigid polyurethane (RPU) foam with expandable graphite (EG) to prepare flame retarded composites. A synergistic effect between nano Zr-AMP and EG on flame retarded and physical-mechanical properties of RPU foam was observed. The RPU foam containing 3 wt% nano Zr-AMP and 12 wt% EG (RPU/12EG/3Zr-AMP) achieved a UL-94 V0 rating with a limiting oxygen index of 30.5%. RPU/12EG/3Zr-AMP showed a 74.4% reduction of peak heat release rate relative to untreated RPU foam. Furthermore, RPU/12EG/3Zr-AMP possessed lower thermal conductivity compared with the RPU foam with 15 wt% EG (RPU/15EG), while the compressive strength of RPU/12EG/3Zr-AMP was enhanced by 40% compared with the one of the latter. The thermogravimetric analysis showed that the simultaneous addition of nano Zr-AMP and EG had a positive impact on improving the char yield and thermal stability of RPU foam. Image 1 • Zr-AMP was synthesized and evaluated as a novel phosphorus and nitrogen-containing flame retardant for RPU foam. • Zr-AMP nanoparticles improved compressive strength, thermal stability and flame retardancy of RPU foam. • Zr-AMP nanoparticles and EG showed good synergistic effect on flame-retardant and physico-mechanical properties in RPU foam. [ABSTRACT FROM AUTHOR]