We have developed a magnesia (MgO)-supported iron-copper (FeCu) catalyst to accomplish the growth of single-walled carbon nanotubes (SWNTs) using carbon monoxide (CO) as the carbon source at ambient pressure. The FeCu catalyst system facilitates the growth of small-diameter SWNTs with a narrow diameter distribution. UV-vis-NIR optical absorption spectra and photoluminescence excitation (PLE) mapping were used to evaluate the relative quantities of the different (n,m) species. We have also demonstrated that the addition of Cu to the Fe catalyst can also cause a remarkable increase in the yield of SWNTs. Finally, a growth mechanism for the FeCu-catalyzed synthesis of SWNTs has been proposed.