Two different types of phosphorus and bromine containing diester monomers, one containing a oxygen-phosphorus-oxygen bond and the other containing a carbon-phosphorus-carbon bond, were synthesized and their chemical structures were characterized by $^1H-NMR$ spectroscopy. Thus, bis(2,6-dibromo-4-methoxycarbonylphenoxy)phenylphosphine oxide(III) was synthesized by bromination of methyl-4-hydroxybenzoate followed by the reaction with phenylphosphonic dichloride. Bis(4-methoxycarbonylphenyl)bromophenyl phosphine oxide(VII) was synthesized by Grignard reaction of 4-bromotoluene with phenylphosphonic dichloride followed by oxidation, esterification, and bromination. Monomer III did not give a high molecular weight polyester when polymerized with ethylene glycol due to decomposition to starting materials via transesterification which was confirmed by thin layer chromatography. The conventional two step melt polymerization of the monomer VII and dimethylterephthalate with ethylene glycol, however, produced high molecular weight copolyesters. Transition behavior of copolyesters was investigated by differential scanning calorimetry and their flame resistance was estimated from % char residue obtained from thermogravimetric analyzer. Copolyesters derived from monomer VII exhibited high % char yield, which is strong evidence of improved flame retardancy.