Abstract: A method for making ultra-fine carbon fibers and activated ultra-fine carbon fibers includes core-sheath melt-spinning to produce a core-sheath fiber including a sheath made of polyolefin and a carbon-containing polymer, and a core made of a polyolefin polymer. Under control of temperature, the core-sheath fiber is stabilized as a carbon-containing precursor fiber. The stabilized core-sheath fiber is carbonized in nitrogen at 600 to 1500 degrees Celsius and converted into ultra-fine carbon fibers with a diameter of 20 to 800 nm. Then, the ultra-fine carbon fibers are activated in carbon dioxide, steam, air or any combination thereof at 600 to 1500 degrees Celsius to form activated ultra-fine carbon fibers.