The growth mechanism and influence of synthesis parameters on the properties of single-walled carbon nanotubes (SWNTs) produced by ferrocene vapor decomposition in a carbon monoxide atmosphere have been investigated in detail by a combined study of Raman and UV-vis-NIR absorption spectroscopy and transmission electron microscopy (TEM). CO2 plays an essential role in selective etching of small diameter nanotubes and the purification of SWNTs. This etching effect is beneficial to narrow the diameter distribution and to control the average diameter of SWNTs. Increasing the synthesis temperature results in the formation of larger catalyst particles due to a higher agglomeration rate, thereby forming larger diameter nanotubes. Decreasing the CO flow rate, and thus lengthening the agglomeration time, also provides the possibility to enlarge the diameter of SWNTs. Therefore, by varying the growth parameters, the mean diameter of SWNTs can be effectively changed from 1.2 to 1.8 nm to satisfy the needs of various applications.