Melamine salt of pentaerythritol phosphate (MPP) was synthesized through the reaction between phosphoric acid, pentaerythritol and melamine with molar ratios 0.8:0.2:0.6. MPP was mixed with polypropylene (PP) at different weight percentages. The advantages of the new way of preparing MPP appeared in the thermal stability and flammability properties of PP composites. TGA results showed that MPP increased the thermal stability of PP to high temperatures. UL94v test indicated that PP composites can achieve VO at 20 % MPP loading level. Another remarkable advantage of the new method of preparing MPP is the sample containing 25 % MPP loading level could withstand without ignition when the flame was applied continuously for 30 s. Limiting oxygen index (LOI) of PP increased to 36.8 % in PP/25 % MPP composite. Cone calorimeter (CC) results showed that addition of MPP to PP greatly reduced the peak of heat release rate and total heat release values. FTIR gas analyser connected to CC showed that the addition of MPP to PP caused massive reductions in the production rates of CO and CO2 gases of PP. Fractional effective dose (FED) calculations indicated that combustion products of PP composites are less toxic than pure polymer. LOI and UL94v tests were repeated after one year on the same samples and they presented nearly the same results. The new data of MPP is expected to increase its industrial applications. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.