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[Display omitted] • A phosphorus/nitrogen-containing hyperbranched epoxy resin (HPITT) was simply synthesized. • Both flame retardancy and toughness of epoxy thermosets were enhanced by HPITT. • EP/HPITT thermosets maintained excellent T g and T 5%. • HPITT was a prospective and versatile modifier for epoxy thermosets. Concurrently enhancing the flame retardancy and toughness without sacrificing strength, thermal decomposition temperature, and glass transition temperature (T g) of epoxy thermosets is still challenging. Herein, a new phosphorus/nitrogen-containing hyperbranched epoxy resin (HPITT) was synthesized from itaconic anhydride, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, piperazine, and triglycidyl isocyanurate by a facile method. The HPITT was then utilized for synergistic flame retarding and toughening modification of epoxy thermosets. Upon the introduction of 10 wt% HPITT, the epoxy thermosets reached a limiting oxygen index (LOI) of 31.5 % and UL-94 V-0 rating. Meanwhile, the peak heat release rate (PHRR) and total smoke production (TSP) decreased by 32.3 % and 27.7 % when compared to neat epoxy thermosets, respectively. Moreover, the synergistic flame retarding functions of phosphaphenanthrene, piperazine and isocyanurate segments in HPITT were demonstrated by combined gas and char phase residue analyses. Compared to neat epoxy, features such as the impact strength, tension strength, and flexural strength of the modified epoxy thermosets increased by respectively 53.9 %, 29.2 %, and 58 % owing to intramolecular cavities, rigid segments, and high crosslinking density supplied by HPITT. Moreover, the modified epoxy thermosets still maintained excellent T g and thermal decomposition temperature. In sum, an attractive strategy for concurrently enhancing flame retardancy and toughness of epoxy thermosets was proposed, useful for future advanced applications in adhesives, coatings, and composite materials. [ABSTRACT FROM AUTHOR]