We have found that the simple introduction of Fe into a conventional NiCo catalyst enables the production of single-wall carbon nanotubes (SWNTs) with controlled diameter distributions by laser ablation. The diameter of as-grown SWNTs can be made larger by increasing the atomic percentage of Fe in the target without changing any other synthesis parameters. We think that Fe helps increase the aggregation rate of metal in laser ablation and enlarges the crystal lattice constant of metal catalysts because of a change in the crystal structure from Ni to Ni3Te, NiFe, and NiFe2, which then results in the enlargement of metal particles and the growth of larger-diameter SWNTs.