A novel flame-retardant chain extension agent (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-3,5-triglycidyl isocyanurate defined as DOPO-TGIC was synthesized, and its chemical structure was well characterized and confirmed. The effect of DOPO-TGIC as a synergistic agent on the flame retardancy, rheology, thermal and mechanical properties of polyamide 6/aluminum diethylphosphinate (PA6/AlPi) composites were investigated in details. The results demonstrated that PA6/AlPi/DOPO-TGIC composites (1.6 mm) successfully passed UL-94 V-0 rating with the limiting oxygen index value of 30.5% when the total loading amount of AlPi/DOPO-TGIC was 10 wt%. In order to achieve the equal flame-retardant level, the individual AlPi was required 14 wt%. The incorporated DOPO-TGIC improved the complex viscosity of PA6/AlPi/DOPO-TGIC composites due to the chain extending reaction between epoxide groups in DOPO-TGIC and the terminal groups of PA6 matrix. The mechanical performance of PA6/AlPi/DOPO-TGIC composites was also improved compared with that of PA6/AlPi composites. DOPO-TGIC stimulated to the formation of more sufficient and compact char layer during combustion. The higher melt viscosity and compact char layer of PA6 composites effectively constrained the volatilization of flammable gases, thus the heat release was reduced. Consequently, the introduction of DOPO-TGIC simultaneously enhanced the flame retardant and mechanical properties of PA6/AlPi/DOPO-TGIC composites compared with that of PA6/AlPi composites. POLYM. ENG. SCI., 59:E206-E215, 2019. (c) 2018 Society of Plastics Engineers