Water tree resistance of crosslinked polyethylene (XLPE) and of low density polyethylene (LDPE) were compared in order to elucidate whether the crosslinking influences or not, the water tree propagation in polymer insulation. To avoid possible errors due to by-products from chemical crosslinking, this study used irradiation crosslinked polyethylene. Irradiation conditions (high dose rate, vacuum) were carefully chosen in order to minimize the errors due to concomitant oxidation. The results obtained indicate that there is no significant difference caused by crosslinking, between water treeing lengths in XLPE and LDPE.