A novel phosphorus-containing curing agent, 2-(6-oxid-6H-dibenz < c,e >< 1,2 > oxaphosphorin-6-yl) phenol formaldehyde novolac [OD-PN], was prepared from phenol formaldehyde novolac resin (PN) and a reactive 2-(6-oxid-6H-dibenz < c,e >< 1,2 > oxa-phosphorin-6-yl)chloride (ODC) while ODC was synthesized through reaction between o-phenylphenol and phosphoryl trichloride. The compound (OD-PN) was used as a reactive flame-retardant in o-cresol formaldehyde novolac epoxy resin (CNE) for electronic application. Owing to the rigid structure of ODC and pendant P group, the resulted phosphorus containing epoxy resin exhibited better flame retardancy, higher glass transition temperature, and thermal stability than the noncyclic P-containing curing agent or the bromine containing flame-retardant epoxy resin. UL 94-VO rating could be achieved with a phosphorus content of as low as 1.21% (comparable to bromine content of 6%) in the cured resin, and no fumes and toxic gas emission were observed, The relationship between the structure and flammability for both phosphorus containing curing agents OB-PN and TP-PN (triphenyl phosphate-phenol formaldehyde novolac reaction product) are also examined.