Two series of novel multifunctional phosphorus-containing epoxy resins (phosphorus content 2 and 4%) were synthesized from the addition reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and the glycidyl ether of cresol formaldehyde novolac (functionality = 12, C-12 and then cured with 4,4'-diaminodiphenyl sulfone (DDS), phenol novolac (PN) or dicyandiamide (DICY). The addition reaction was monitored by high performance liquid chromatography (HPLC) and epoxide equivalent weight (EEW) titration. The proposed structure was confirmed by FTIR and NMR spectra. Thermal properties of cured epoxy resins were studied using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and thermal gravimetric analysis (TGA). The flame-retardancy of cured epoxy resins was tested by UL-94 vertical test and limiting oxygen index (LOI). These cured epoxy resins exhibited high glass transition and high thermal stability and achieved UL-94 vertical tests of V-0 grade (nonflammable).