A main-chain phosphorus-containing diepoxide, bis-glycidyloxy phenylphosphate (BGPP), was synthesized, then blended with various contents of a 4,4'-diglycidylether of bisphenol A (DGEBA)-type epoxy resin, DER 331. The curing reactions of the blended resins with diethyltriamine (DETA) and the thermal properties of the cured specimens were investigated with an infrared spectrophotometer, a differential scanning calorimeter, and a thermogravimetry analyzer. The activation energy is 51.2 kJ/mol for the BGPP-DETA curing system, and it increases with the content of DER 331 for the BGPP-DER 331-DETA systems. The average apparent decomposition activation energies of the thermosets with phosphorus-containing epoxide are from 147.3 to 165.1 kJ/mol, which are higher than that. of the DER 331-DETA system (95.8 kJ/mol). The BGPP-DETA system shows a relatively higher char yield and limiting oxygen index (LOI) value, 29-31, but the thermoset with BGPP results in a decrease in the T-g.