The effect of 5-200 nm of vacuum ultraviolet (VUV) irradiation on the structure and tensile properties of polytetrafluoroethylene (PTFE) films was investigated. The change in structure before and after VUV irradiation was evaluated with differential scanning calorimetry (DSC) and electron spin resonance (ESR) analysis. DSC analysis results showed that the melting point, melting enthalpy, and crystallization enthalpy increased and the maximum crystallization temperature decreased a little with increasing VUV dose. It was deduced from the DSC data that the molecular weight of the PTFE films decreased and the destruction of the crystal structure took place under VUV irradiation. The ESR results showed that the radicals formed under VUV irradiation were chain-end radicals and peroxy radicals, which may have been formed due to the scission of carbon-to-carbon bonds. The tensile fracture strength and elongation decreased with increasing VUV irradiation dose. Under the same irradiation dose, the tensile properties were more sensitive to the lower VUV intensity. The decrease in the molecular weight and the destruction of crystal structure might have been the major reasons for the deterioration of the tensile properties. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1494-1497, 2004.