We propose a novel technique to prepare carbon fibers from cotton fibers while retaining the unique hollow structure of these natural fibers. We were able to introduce micropores and mesopores into the hollow structure by using zinc nitrate as an activating agent. We revealed that the porosity of the obtained carbon fibers and the pore size increase as the concentration of the activating agents is increased. Moreover, the obtained carbon fibers were applied in electric double-layer capacitors and found to exhibit excellent performance due to the rapid ion transport through the hollow cores and nanopores.