Abstract: A novel compound with phosphaphenanthrene and triazole groups (DTZ) for highly efficient flame-retardant epoxy resin was synthesized with p-phthalaldehyde, 3,5-diamino-1,2,4-triazole and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Its chemical structure was characterized by FTIR, NMR, GPC and elemental analysis. The thermal properties of epoxy thermosets were investigated by TGA and DSC analyses. The flame-retardant and mechanical properties were evaluated by limiting oxygen index, UL-94, cone calorimeter and tensile tests. Moreover, the flame-retardant mechanism of DTZ was particularly investigated by analyzing the pyrolysis and thermo-oxidative degradation behaviors, along with the morphology and structure of residual char of epoxy thermoset. It reveals that the incorporation of DTZ decreases the initial degradation temperature and glass transition temperature, while increases the char yield at elevated temperature and tensile strength for thermoset. DTZ remarkably improves the flame retardancy of epoxy thermoset. With the incorporation of 6wt% DTZ, the modified epoxy thermoset achieves an limit oxygen index (LOI) value of 33.5% and passes UL-94 V-0 rating. Moreover, the peak heat release rate and total heat release in cone calorimetry test decrease by 21.8% and 18.2%, respectively. DTZ shows high flame-retardant activity not only by diluting combustible volatiles and quenching active radicals in the gas phase, but also by promoting the char formation in the condensed phase.