Phosphorus-containing epoxy resins were synthesized by o-cresol-novolac epoxy resin (CNE) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) catalyzed by triphenyl phosphine (TPP). The reaction was characterized by FTIR, differential scanning calorimeter(DSC), Advanced Polymer Chromatography (APC) and HCl/acetone titration method. The results showed that the reaction of CNE and DOPO completed in 2 h at 130 degrees C with 1.5%TPP and the activation energy reduced from 162 kJ/mol to113 kJ/mol. The thermal properties and flame retardancy of the phosphorus-containing epoxy resins cured by phenol formaldehyde novolac resin(PN) were investigated by simultaneous thermal analyzer, cone calorimeter, limiting oxygen index (LOI) and vertical burning test (UL-94). The results revealed that the DOPO groups decomposed into phosphoric or polyphosphoric acid in air and decomposed into PO free radicals in nitrogen. The epoxy thermoset with 1.5% phosphorus content achieved UL94 V-1 rating and LOI 27% and the epoxy thermoset with 2.4% phosphorus content achieved UL94 V-0 rating and LOI 31%. The morphology of residues after cone calorimeter test was observed by scanning electron microscope (SEM). The results showed that the char of DOPO-CNE(2%P)/PN and DOPO-CNE(3%P)/PN present a compact and continuous surface. (C) 2016 Elsevier B.V. All rights reserved.