Low-molecular-weight PE such a highdensity PE (HDPE) is often blended with UHMW-PE to improve its melt processability. [18-28] For instance, high-strength UHMW-PE/HDPE blended fibers were prepared by melt-spinning. [23,28] Interestingly, the blending of UHMW-PE induces the development of mechanical properties originating from the composite structure formation for melt-proces sing. [19,21,24,25,27,29] This is because the difference in stress transmissibility between HDPE chains and UHMW-PE chains in the molten state based on the difference in relaxation time. [13] UHMW-PE chains transfer the applied stress via entanglements owing to its long relaxation time [30] and form highly oriented crystals (extended-chain crystals) even in melt-proces sing, [3-6,11,12,16,17] whereas HDPE chains are disentangled and provide lamellar crystals (folded-chain crystals). In this study, we attempted to prepare high-performance PE fibers by melt-spinning and subsequent drawing in the semimolten state of HDPE combined with UHMW-PE by introducing a structural composite effect based on the difference in relaxation time. The as-spun fiber prepared by melt-spinning was drawn just below the melting temperature (semimolten state) to induce highly oriented chains derived from UHMW-PE. The formation of highly oriented crystals derived from UHMW-PE contributes to high tensile strength, but energy absorption ability is poor. On the other hand, the formation of the lamellar structure derived from HDPE induces energy absorption, i.e., high toughness, derived from the deformation of lamellar crystals. Hence, the composite crystalline structure having different structural functions can be formed by the drawing of a HDPE/ UHMW-PE blended fiber in the semimolten state, resulting in a PE fiber with both high tensile strength and high toughness. 2. Experimental Section 2.1. Materials HDPE reactor powder having a weight-average MW of 1.0 × 10 5 (J-REX, Japan Polyolefins Co., Ltd., Japan) and UHMW-PE reactor powder having a viscosity-average MW of 1.15 × 10 6 (HI-ZEX MILLION 145M, Mitsui Chemicals, Inc., Japan) were used. UHMW-PE powder was blended with HDPE powder at the blend ratio of 0-30 wt% (Table 1). Before melt-spinning, High-strength and high-toughness polyethylene (PE) fibers are prepared by the melt-spinning and subsequent drawing in the semimolten state of highdensity PE (HDPE) combined with ultrahigh-molecular-weight PE (UHMW-PE). As-spun fibers are prepared by the melt-spinning of HDPE/UHMW-PE blended powder with 0-30 wt% UHMW-PE. The as-spun fiber is drawn at 100 (solid state) and 130 °C (semimolten state). Well-arranged stacked lamellae with highly oriented crystals are formed based on the difference in relaxation time for the drawn fiber with 30 wt% UHMW-PE prepared at 130 °C in the semimolten state. This composite structure results in a PE fiber with high tensile strength (1.11 GPa) and high toughness (297 MJ m −3).