The improvement of flame-retardation of thermosetted epoxy-amine resins was attempted by chemically incorporating phosphorus-containing reagents. By reacting 4,4’-diglycidylether of bisphenol A (DGEBA) with dialkyl(or aryl) phosphate, it was possible to chemically modify the epoxy resin and then cure it in the presence of 4,4’-diaminodiphenylsulfone (DDS) to obtain epoxy-amine resin with good flame-retardant and thermal stability behaviors. The quantitative aspect of the addition of dialkyl(or aryl) phosphate onto glycidyle oxiranes was evaluated by elemental analysis of the modified epoxy-amine resins. Flammability and thermal behaviors of modified DGEBA/DDS resins depend on the nature of phosphate groups (the best flame-retardation was observed on resins bearing phenyl phosphate groups) and their concentration in the material. In relation to DGEBA/DDS samples containing additives of the same structure [trialkyl(or aryl) phosphate], cured resins incorporating chemically bonded phosphate groups show a better flame-retardation. On the contrary to the nonmodified DGEBA/DDS [with or without trialkyl(or aryl) phosphate as additive], combustion of modified DGEBA/DDS resins is accompanied by formation of intumescent char. Chemical modification of DGEBA by dialkyl(or aryl) phosphates can be carried out in situ during the curing of epoxy resins without change in the fire behavior.