2-(6-Oxido-6H-dibenz [c,e][1,2] oxaphosphorin-6-yl) dimethyl itaconate (ODOP-DI) was prepared by the addition reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and dimethyl itaconate while DOPO was synthesized through multistep reaction from o-phenylphenol and phosphorus trichloride. The compound (ODOP-DI) was used as a reactive flame retardant in polyethylene 1,4-terephthalate (PET) and polyethylene 2,6-naphthalate (PEN) for film, fiber, and electronic applications. The thermal properties of the resulted copolyesters were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Owing to the incorporation of the rigid structure of DOPO and pendant P groups, the resulted phosphorus containing copolyesters exhibited better flame retardancy, higher char yield, and thermal stability than homopolymers of PET and PEN. UL 94-VO rating could be achieved with a phosphorus content of as low as 0.75% for PET, 0.5% for PEN, and no fume and toxic gas emission were observed.