The chain initiation reaction mechanism of hexachlorocyclotriphosphazene, which was synthesized by phosphorus pentachloride and ammonium chloride was investigated by using the density functional theory (DFT) of quantum chemistry. At the GGA-HCTH/DND level, the geometry optimizations of reactants, intermediates, the transition states and the products were performed. All of the transition states were verified by the frequency analysis method. The results showed that the chain initiation reaction was a very complex procedure so there were various intermediates and transition states along reaction paths and the product was [Cl₃P…N…PCl₃]^＋ which was agreed well with experimental results. This reaction was an endothermal process, and it only took place when heated. For this reaction paths, IM2→TS2→IM3＋HCl had the lowest energy barrier, only 12.18 kJ/mol|while the fifth procedure (IM5＋PCl₅→TS5→IM6＋HCl) was the rate-controlling step, and it had the highest energy barrier 174.83 kJ/mol.

Key words: hexachlorocyclotriphosphazene, chain initiation, reaction mechanism, density functional theory, transition state