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The continuous updating and iteration of electronic devices brings about a significant accumulation of heat. It is urgent to enhance the thermal properties of polymer‐based composites to alleviate the heat dissipation problem in the microelectronics industry. In this paper, the hot drawing process was introduced to increase the crystallinity of ultra‐high molecular weight polyethylene (UHMWPE) fiber and improve its thermal conductivity. The ultra‐high molecular weight polyethylene fiber/epoxy resin (UHMWPE fiber/Epoxy) composites with high thermal conductivity were prepared by compositing UHMWPE fiber with epoxy resin under vacuum conditions. Due to the good crystallinity of the UHMWPE fiber (86%), an efficient thermal conduction path is provided for phonons in the axial direction along the fibers. The results show that the thermal conductivity of the composites reaches a maximum of 3.51 W/mK (in‐plane), which is 17.47 times higher than that of the epoxy resin (0.19 W/mK). The thermography pictures indicate that the composites have better heat transfer capacity with an increase in the draw ratio (λ). In addition, the composites also have the characteristics of low density and low electrical conductivity. These researches will provide a new idea for the preparation of all‐polymer composites with high thermal properties. Highlights: A hot drawing process was introduced to increase the crystallinity of UHMWPE fiber for improving thermal conductivity.High crystallinity facilitates the reduction of phonon scattering and the transmission of thermal energy by phonons.The obtained polymer composites exhibit an ultra‐high in‐plane thermal conductivity. [ABSTRACT FROM AUTHOR]