Poly(butylene terephthalate) (PBT) copolyesters modified with seven compositions of p-acetoxybenzoic acid (PABA) ranging from 10 to 70 mol % were prepared. The X-ray diffraction patterns, the polarizing microscopy behaviors, and thermal analysis showed that the modified PET contained more PABA homopolymer units (PABA-PABA) than PBT-PABA units in the copolyesters. On increasing PABA mole percentage, PET crystallinity decreased and thermal stability increased. It was found that although the PET copolyesters did not exhibit a clear liquid crystalline texture like the copolyester of poly(ethylene terephthalate) modified with PABA did, the PET copolyester containing 70 mol % of PABA exhibited the typical shear thinning behavior of a liquid crystalline polymer.