Electron beam irradiation of polytetrafluoroethylene (PTFE) has been per formed in vacuum both at elevated temperature above the melting point of PTFE and at room temperature for comparison. The changes in the chemical structure were studied by Fourier-transform infrared (FTIR) spectroscopy. The formation of double bonds in PTFE was confirmed as a result of irradiation in vacuum. Moreover, trifluoromethyl (CF3) branches were detected. The concentration of CF3 branches was found to be much higher in irradiation at elevated temperature than in room temperature irradiation. The CF3 branches are assumed to cause the reduced crystallinity indicated by the transparency of PTFE specimens irradiated in the molten state.