Starting with 3,3',4,4'-biphenyltetracarboxylic dianhydride and methyl aminobenzoate, we synthesized a novel rodlike imide-containing monomer, N,N'-bis[p-(methoxy carbonyl) phenyl]-biphenyl-3,3',4,4'-tetracarboxydiimide (BMBI). The polycondensation of BMBI with dimethyl terephthalate and ethylene glycol yielded a series of copoly(ester imide)s based on the BMBI-modified poly(ethylene terephthalate) (PET) backbone. Compared with PET, these BMBI-modified polyesters had higher glass-transition temperatures and higher stiffness and strength. In particular, the poly(ethylene terephthalate imide) PETI-5, which contained 5 mol % of the imide Moieties, had a glass-transition temperature of 89.9 degreesC (11 degreesC higher than the glass-transition temperature of PET), a tensile modulus of 869.4 MPa (20.2 % higher than that of PET), and a tensile strength of 80.8 MPa (38.8 % higher than that of PET). Therefore, a significant reinforcing effect was observed in these imide-modified polyesters, and a new approach to higher property polyesters was suggested.