4-Hydroxy-2,3,5,6-tetrafluorobenzoic acid/6-hydroxy-2-naphthoic acid copolymers (FHBA/HNA copolymer) with different copolymer compositions were prepared and the influence of FHBA residue on the thermal properties and structures of the copolymers were investigated. Introduction of FHBA decreased the crystal/nematic phase transition temperatures (T(CN)s) of the FHBA/HNA copolymers. T(CN)s Of the copolymers were in the temperature range between 200 and 250 degrees C, depending on the copolymer composition. They are approximately 40 degrees C lower than those of 4-hydroxybenzoic acid/HNA copolymers. FHBA/HNA copolymers exhibited low crystallinity, and annealing treatment hardly influenced the crystalline natures. FHBA residue possibly interferes with the recrystallization during annealing.