The thermal degradation and melt viscosity of ultra-high-molecular-weight poly(ethylene terephthalate) (UHMW-PET) with an intrinsic viscosity exceeding 2 dl g(-1) were studied in the melt phase under a nitrogen atmosphere. The degradation rate increased with the molecular weight of the polymer. The high degradation rate of UHMW-PET could be interpreted by the differences in the terminal-end-group concentrations. The activation energy for the degradation of UHMW-PET with an intrinsic viscosity (IV) of 2.3 dl g(-1) was 41 kcal mol(-1). The effect of molecular-weight change on the thermal degradation of UHMW-PET could be simulated by an empirical formula. The melt viscosity of UHMW-PET with M(w), = 2.3 x 10(5) at 300 degrees C under zero shear was 8 x 10(5) P and the empirical formula for calculation of the melt viscosity was eta(0) = 1.45 x 10(-17) M(w)(4.23).