A flame retardant, tri-(3-DOPO-2-hydroxypropan-1-yl)-1, 3, 5-triazine-2, 4, 6-trione (TGIC-DOPO) containing both phosphaphenanthrene and triazine-trione groups, is introduced into diglycidyl ether of bisphenol-A (EP) thermosets respectively cured by 4,4'-diamino-diphenyl methane (DDM), 4,4'-diaminodiphenyl sulfone (DDS), and m-phenylenediamine (m-PDA). TGIC-DOPO exhibits excellent flame-retardant effects in EP/DDM compared with EP/DDS and EP/m-PDA. The thermoset cured with DDM with only 4%TGIC-DOPO reaches UL94 V-0 rating and possesses a limited oxygen index (LOI) value of 35.6%. The macroscopic and microscopic morphologies of the residues reveal that a cage-like char crown envelops the fire, thus hindering oxygen from permeating inside and inhibiting the release of PO and phenol free radicals with quenching effect. The char-cage hindering effect is the main reason for the high LOI values. TGIC-DOPO in EP/DDM not only locks more carbon components in condensed phase but also facilitates to release more pyrolyzed PO and phenol free radicals. The concentrated release of PO and phenol free radicals exert a strong quenching effect, which is the main mechanism for the high flame-retardant rating with the addition of relatively low amount of TGIC-DOPO. Therefore, the integration of char-cage hindering and free radical quenching effects enables excellent flame retardancy to epoxy thermosets. (C) 2015 Elsevier Ltd. All rights reserved.