Thermal interface materials (TIMs) with high through-plane thermal conductivity are urgently desired to avoid overheating of high-power density electronics. Introducing and aligning fillers in polymer matrixes via magnetic field is a promising method to improve the thermal conductivity of the polymer. However, either the fillers need to be modified with magnetic particles or a strong magnetic field is needed for good alignment in high filler content. This prevents further improvement of the through-plane thermal conductivity. Herein, mesophase pitch-based carbon fibers (MPCFs) with a content as high as 76 wt.% are aligned vertically in water-soluble polyvinyl alcohol (PVA) under a low magnetic field (~ 0.4 T), forming a vertically aligned MPCF (VAMPCF)/PVA composite with an extraordinary through-plane thermal conductivity of 86 W/(m·K), which is higher than that of many alloys. In addition, both theoretical and experimental results demonstrate that the critical intensity of the magnetic field needed for good alignment of the fillers depends on their size and magnetic susceptibility. Furthermore, the water solubility of PVA makes it easy to recycle MPCFs. This study offers an inspired venue to develop excellent and eco-friendly TIMs to meet ever increasing demand in heat dissipation for electronics.