The diameter of high-pressure CO (HiPco) single-wall carbon nanotubes (SWNTs) has been enlarged by treating them with nitric acid and oxygen, and the diameter enlargement is related to the nitric acid refluxing time. Raman spectra (exciting wavelength 488 nm) and transmission electron microscopy showed that the 0.8-1.2 nm diameter distribution of HiPco SWNTs could be narrowed to one centered around 1.2 nm when the nitric acid treatment time was I h. Significantly, SWNTs of 1.37-2.4 nm in diameter were discovered when the nitric acid treatment lasted more than 2 h.