In an attempt to improve thermal and flame-retardant properties of epoxy resins efficiently, a new reactive phosphorus-containing curing agent called 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-yl-(phenylimino)-(4-hydroxyphenyl)me-thane (DOPO-PHM) was synthesized and was combined with 4,4 '-diaminodiphenyl methane (DDM) to co-cure epoxy resins (E51), which covalently incorporated halogen-free DOPO organ groups into the epoxy networks. The chemical structure of this curing agent was confirmed by FTIR, 1D, and 2D NMR spectra. A reaction mechanism during the preparation was proposed, and the electron effect on the stabilization of the carbocation was discussed. Various DDM/DOPO-PHM molar ratios were used to get the materials with different phosphorus contents. Their dynamic mechanical, thermal, and flame-retardant properties were evaluated by dynamic mechanical thermal analysis, thermogravimetric analysis, and limiting oxygen index (LOI) respectively. All samples had a single T-g, showing that these epoxy resins were homogeneous phase for long-term use in spite of adding DOPO-PHM. Both char yields (under nitrogen and air atmospheres) increased with the increasing of phosphorus content and the LOI values increased from 24.5 for standard resin to 33.5 for phosphorus-containing resins, indicating the significant enhancement of thermal stability and flame retardancy. POLYM. ENG. SCI., 54:1192-1200, 2014. (c) 2013 Society of Plastics Engineers