Thermal analysis has been carried out on aromatic polyesters(i.e. poly(ethylene 2,6-naphlhalate) and poly(butylene 2,6-naphthalate)) over a range of 270 to 31$0^{circ}C$, above the melting temperature, without the addition of catalysts and stabilizers. Degradation products of polyesters were characterized by gas chromatography and mass spectrometer. It was concluded that poly(ethylene 2,6-naphthalate) decomposed by a random scission of ester links involving a six centered cyclic transition state yields vinyl naphthoate and naphthoic acid and poly(butylene 2,6-naphthalate) decomposed by two random scission of ester links gives 1-butenyl naphthoate and naphthoic acid.