Positron annihilation lifetime (PAL) as well as Doppler broadening (DB) has been measured for unirradiated and three different neutron-irradiated polyethylenes. The variation of the ortho-positronium (o-Ps) lifetime, tau (3), its intensity, I-3, and the S-parameter with temperature up to 150 degreesC is studied for all samples. At room temperature, tau (3) is almost constant up to 165 Gy while I-3 increases slightly with increasing radiation doses. This may be attributed to chain scission and cross-linkings caused by radiation. An abrupt increase in I-3 around 120 degreesC is observed which corresponds to start of melting of crystals.