The structural change in poly(hexafluoro-1,3-butadiene) that takes place during heat treatment was investigated by measuring its i.r. spectra. The polymer obtained from hexafluoro-1,3-butadiene (HFBD), produced by t-C4H9OCs initiation in toluene, showed excellent thermal properties, such as a high thermal decomposition temperature and a low thermal expansion coefficient. The thermal properties of poly(HFBD) were found to be similar to those of conventional thermosetting polymers, since the thermal expansion coefficient of poly(HFBD) decreased after the heating process. The absorption at approximately 2970 cm-1, which was assigned to traces of initiator (t-C4H9-), disappeared after heating at 200 or 400-degrees-C for 10 h. The absorption at approximately 1710 cm-1, which was assigned to the -CF=CF- bond in the main chain, and the peak at 1840 cm-1, both increased after heating at 400-degrees-C for 10 h. It is suggested that the chemical structure of the polymer was changed during the heating process. Furthermore, the radical species which was produced by cleavage of the bonds with heating at high temperatures might react with the -CF=CF- bonds in the polymer chain to yield crosslinked species.