The influence of the polymerisation degree on the thermal properties of poly(tetrafluoroethylene) (PTFE) irradiated with 1.5 MeV electrons under various conditions was investigated. The polymerisation degree of radiation-degraded and radiation-crosslinked PTFE was determined by the analysis of chemical structure such as end groups and branching points using F-19 NMR and IR methods. The melting heat and temperature observed in the as-polymerised and melt-crystallised states deviate from each other for high molecular weight PTFE. The differences vanish at polymerisation degree X-n < 1600 produced by irradiation with dose of 100-200 kGy. The representation of melting temperature of the melt-crystallised state as function of X-n permits the estimation of the equilibrium melting temperature T-m(o) as 605 K, which agrees with values given in the literature for the extended-chain crystals. We conclude that mainly extended-chain crystals were formed for PTFE of X-n less than or equal to 1600 at the melt-crystallisation, whereas PTFE of X-n > 1600 crystallises as folded-chain crystals. The lamellar thickness l(c) of the folded-chain crystals can be estimated as about 100 nm. The irradiation of PTFE in the melt results in crosslinking and branching with a strong influence on the thermal properties.