Abstract Ammonium polyphosphate (APP) form II was prepared by heating of a mixture of ammonium orthophosphates (triammonium, diammonium, and monoammonium orthophosphates) and urea at 280 to 300 °C under wet ammonia, which was made by passing air through 3 to 5% aqueous ammonia. The following reaction process is proposed: Ammonium orthophosphate → APP form I → Amorphous APP → APP form II. The addition of APP form II shortened the preparation time significantly. A wider range of concentrations (1 to 8%) of aqueous ammonia was available to prepare APP form II. The average particle size of the obtained APP form II was larger than 5 μm.