Thermal degradation behavior of the three homologous polyesters, poly(ethylene terephthalate)(PET), poly(trimethylene terephthalate)(PTT) and poly(butylene terephthalate)(PBT) were investigated by using thermogravimetric analysis(TGA) and pyrolysis-gas chromatography-mass spectrometry(PGC-MS). From the results of isothermal thermogravimetry, it was revealed that PTT has higher activation energy of thermal degradation than the other two polyesters. In PGC-MS analysis, the highly volatile products of the polyesters such as acetaldehyde and acetic acid evolved in the early stage of pyrolysis as common degradation products. Benzoic acid, methyl ester of benzoic acid, terephthalic acid and higher oligomers were produced as the main degradation products of poly(alkylene terephthalate)s at longer pyrolysis times. In isothermal pyrolysis, PTT showed the fastest thermal degradation rate among the three polyesters.