A study on the contribution of thermal volume expansion to electrical properties is carried out for high-density polyethylene (HDPE)/carbon black (CB) composites irradiated by an electron beam. The results show that the volume expansion obviously generates the positive temperature coefficient (PTC) characteristic of resistivity for unirradiated HDPE/CB composites, but the contribution of volume expansion is decreased for crosslinked HDPE in the composites by electron beam irradiation. A higher degree of crosslinking produced by irradiation in the molten state limits the movability of HDPE chains and CB particles so effectively that it decreases the PTC intensity, which is compared with that irradiated at room temperature. It is suggested that the differences in the resistivity-temperature behavior are not explained satisfactorily on only the basis of the thermal volume expansion, and the decreased movability of HDPE chains and CB particles are believed to be the most fatal factors in lowering the PTC effect.