High alignment and densification of carbon nanotubes (CNTs) are of key importance for strengthening CNT fibers, whereas direct stretching has a very limited effect when CNTs are highly entangled. We report that by lubricating CNT surfaces with viscous alcohols, the relative motion between CNTs improves because of the reduced sliding energy barrier; thus non-stretched regions are effectively eliminated. Owing to the very efficient optimization of the assembled structure, the stretched CNT fibers exhibited an average tensile strength of 2.33 GPa (1.82 N per tex) and modulus of 70.1 GPa (54.8 N per tex). Other fundamental properties, such as electrical and thermal conductivities, were also remarkably improved. Such a strategy can be readily used for manufacturing high-performance CNT assemblies and composites.