A series of copolyesters based on poly(trimethylene terephthalate) derived from 2,5-thiophenedicarboxylic acid (TFDCA) have been successfully synthesized by the melting polymerization. Microstructures and molecular weights were characterized by H-1 NMR and size exclusion chromatography (SEC), respectively. Meanwhile, thermal properties, crystallization behaviors, and crystal structures were investigated by DSC, TGA, and WAXD. The results confirmed the copolymer compositions were nearly equal to the feed compositions. The weight molecular weights were all higher than 21000 g/mol for all polyesters synthesized. With increasing the TFDCA content, the glass transition temperature decreased slightly due to the weaker rigidity of the thiophene ring. DSC and WAXD results demonstrated that the isodimorphic behavior occurred for poly(trimethylene 2,5-thiophe-nedicarboxylate-co-trimethyleneterephthalate)s (PTTTFs). When TFDCA content was 25%, PTTTF25 exhibited the moderate tensile strength (53.1 MPa). However, the high elongation at break (396%) was observed due to the beta relaxation and strain-induced crystallization.