Abstract: The many uses of carbon nanotubes (CNTs) depend not only on their intrinsic physical properties, but also on their tunable chemical components. Exploring a low-temperature method for the incorporation of phosphorus atoms in the carbon framework is expected to change the chemical properties of CNTs. Here, phosphorus-functionalized CNTs (PCNTs) were prepared by the direct hydrothermal treatment of a CNT-H₃PO₄ mixture at 170℃. The PCNTs had a high phosphorus content of 1.66 at%, a specific surface area of 132 m²·g^-1, and an improved thermal stability with a weight loss peak at 694℃ during oxidation in pure oxygen. They showed good electrocatalytic activity for the oxygen reduction reaction with an onset potential of 0.20 V vs Hg/Hg₂Cl₂, an electron transfer number of 2.60, and a larger current density as well as improved cyclic stability compared with pristine CNTs. PCNTs were also used as conductive scaffolds for the cathode in lithium-sulfur batteries. The cathode delivered an initial discharge capacity of 1 106 mAh·g^-1, a capacity retention of 80% from 0.1 to 1.0 C, and a low decay rate of 0.25% per cycle during 100 cycles.