Gamma-irradiation of carbon nanotube yarns in air has significantly improved the tensile strength and modulus of the yarns, presumably because of an increased interaction between the individual nanotubes. The improvement has been much greater for tightly structured yarns than for loosely structured yarns. Sonic pulse tests have also shown increased sound velocity and dynamic modulus in the carbon nanotube yarns as a result of gamma-irradiation treatment. X-ray photoelectron spectroscopic analyses on progenitor carbon nanotube forests show that gamma-irradiation treatment in air has dramatically increased the concentration of oxygen, for example as carboxyl groups, in the carbon nanotube assemblies in proportion to radiation dose, indicating that carbon nanotubes were oxidized under the ionizing effect of the gamma-irradiation. Such oxygen species are thought to contribute to the interaction between carbon nanotubes and thus to the improvement of carbon nanotube yarn mechanical properties.