We investigated the structure and properties of extruded high-density polyethylene (HDPE) containing 03. wt.％ of multi-walled carbon nanotubes (CNTs). It was found from differential scanning calorimetry measurements that the CNT addition raised the crystallization temperature of HDPE, suggesting that CNT acts as a nucleating agent for HDPE. Furthermore, two-dimensional X-ray diffraction measurements revealed that the CNT addition greatly enhanced the chain orientation of HDPE in the extruded strand, which was pronounced by melt-stretching after passing through the die. CNTs orientated parallel to the flow direction behave like extended chain crystals, and thus accelerate the formation of shish-kebab structure. As a result, the tensile modulus was enhanced by the CNT addition, which was pronounced especially in the high temperature region.