The thermal decomposition stability of PTT resins with different molecular weights was studied in nitrogen and air, respectively. The results showed that PTT resins exhibited a thermo weight-loss process in nitrogen, which might correspond to the thermal degradation of PTT chains. The thermal degradation behavior of PTT in air was quite different from that in nitrogen, which exhibited two thermo weight-loss processes in air. The first decomposition process was produced by the degradation of polymer chains as that in nitrogen. And the second decomposition process was mainly caused by the thermo-oxidative degradation of the products formed during the first decomposition process. The first decomposition process in nitrogen and in air could be taken as the characterizations of the thermal degradation and thermo-oxidative degradation stabilities of PTT, respectively.