This paper explains the main. cause of the formation of microvoids (1µm-several 10sµm) in the insulation of crosslinked polyethylene, and the effect of those on the electrial characteristics of cable. Crosslinked polyethylene cable is produced through the process of extruding, moulding, heating, crosslinking and cooling. Steam is usually used as the heating medium. The first cause of the formation of microvoids is the steam intruding into the insulation at the time of vulcanization, and the insulation of steam cured crosslinked polyethylene cable generally contains microvoids several µm in size and around 106per mm3in number. If a heating medium other than steam should be used, the number of voids can be reduced by around 104per mm3, but it is quite difficult to bring it down to zero. While cable breakdown voltage may scarcely be affected by the size and number of the microvoids at present concerning us, the dimension of bow tie tree induced by the microvoids is approximately proportional to the diameter of voids, and so efforts must be made to reduce the diameter of voids.