Abstract: An epoxy resin was modified with an intumescent flame-retardant system (IFR) based on poly(2-10-hydrogen-9-oxa-phosphaphenanthrenehydroquinone phenyl phosphate) (POPP) and ammonium polyphosphate (APP) at a weight ratio of 1/2,and then cured with m-phenylenediamine to form flame-retardant thermosets (FR-EP). Organic montmorillonite (OMMT) was incorporated into this IFR to obtain a synergistic flame-retardant and smoke-inhibiting effect at an OMMT content of 1.0 wt %. The combining effect of IFR and OMMT on epoxy resin was investigated by means of limiting oxygen index (LOI), vertical burning experiment, thermogravimetric analysis, cone calorimetry and scanning electron microscopy. The results indicated that the POPP/APP/OMMT-modified epoxy thermosets achieved a V-0 classification in the UL 94 vertical burning experiment as well as an LOI of 25.2 vol %. The initial decomposition temperature of the modified epoxy thermosets decreased slightly, but the residual char increased significantly. According to the cone test results, the heat release rate, total heat release and total smoke release all decreased dramatically. Moreover, the combination of POPP, APP and OMMT promoted the formation of a homogeneous and compact char layer on the epoxy thermoset during combustion. In addition, the OMMT was found to enhance the mechanical properties of the epoxy thermosets.