The effect of atmospheric-pressure plasma treatment on the mechanical properties of carbon nanotube (CNT) fibers was investigated. When the CNT fibers were irradiated with RF atmospheric-pressure plasma, it was confirmed that functional groups such as hydroxyl or carboxylic acid groups were generated on the surface of the CNT fibers. The mechanical properties of CNT fibers were improved as a result of the plasma treatment because the functional groups on individual CNTs were able to induce hydrogen bonds between them. After plasma treatment with gas flow of argon (10 lpm) and oxygen (100 sccm), the tensile strength of the pure CNT fiber increased by approximately 40%, from 0.62 N/tex to 0.86 N/tex, and the modulus increased by approximately 20%, from 27.9 N/tex to 32.9 N/tex.