High-thermal-conductivity polymers are very sought after for applications in various thermal management systems. Although improving crystallinity is a common way for increasing the thermal conductivity ( k) of polymers, it has very limited capacity when the crystallinity is already high. In this work, by heat-stretching a highly crystalline microfiber, a significant k enhancement is observed. More interestingly, it coincides with a reduction in crystallinity. The sample is a Spectra S-900 ultrahigh-molecular-weight polyethylene (UHMW-PE) microfiber of 92% crystallinity and high degree of orientation. The optimum stretching condition is 131.5 °C, with a strain rate of 0.0129 s –1 to a low strain ratio (∼6.6) followed by air quenching. The k enhancement is from 21 to 51 W/(m·K), the highest value for UHMW-PE microfibers reported to date. X-ray diffraction study finds that the crystallinity reduces to 83% after stretching, whereas the crystallite size and crystallite orientation are not changed. Cryogenic thermal characterization shows a reduced level of phonon-defect scattering near 30 K. Polarization Raman spectroscopy finds enhanced alignment of amorphous chains, which could be the main reason for the k enhancement. A possible relocation of amorphous phase is also discussed and indirectly supported by a bending test.