AbstractA new synergetic flame retardant system (SFRS) using ammonium polyphosphate/melamine cyanurate (APP/MCA) core-shell particle was synthesized and applied to papermaking. Cationic polyacrylamide (CPAM) was added to improve the adsorption of SFRS in pulp. Higher amounts of SFRS led to higher adsorption by pulp. Handsheets were produced with different amounts of SFRS. Thermal gravimetric analysis (TGA) and limiting oxygen index (LOI) were employed to test fire retardation in these handsheets. TGA and LOI results showed that this new core-shell SFRS is an effective flame retardant that improves the thermal stability of handsheets.