A novel epoxy thermosetting system based on a phosphazene-cyclomatrix network was designed and successfully prepared through the crosslinking reaction of diglycidyl ether of bisphenol-A with trispiro(ethylenediamino)-cyclotriphosphazene. The study of isothermal crosslinking kinetics indicated that the crosslinking process could be simulated by the Kamal's model. The resultant thermosets exhibit excellent flame-retardant properties with a UL94 V-0 flame rating due to the presence of phosphazene-cyclomatrix network, whereas they show a higher thermal stability than the conventional epoxy thermoset. The mechanism study indicated that the phosphazene-cyclomatrix moieties could effectively enhance the flame retardancy by acting in both condensed and gaseous phases to promote the formation of an intumescent phosphorus-rich carbonaceous char layer on the surface of material. Such a char layer could prevent the volatiles from transferring to the surface of the thermosets as well as to shield the heat and oxygen diffusion, thus resulting in the self-extinguishing flame rating. (C) 2015 Elsevier B.V. All rights reserved.