Diameter selective growth of single-walled carbon nanotubes (SWCNTs) is one of the most important challenges for the application of SWCNTs in future high-performance nanodevices. In this investigation, SWCNTs with selective diameters have been synthesized by DC arc discharge method, in which carbon monoxide (CO) mixed with helium were used as buffer gases. The experimental results show that SWCNTs with small diameters are preferentially etched with the increase of CO concentration. However, pure CO as buffer gas can result in the formation of carbon nanoparticles. Appropriate CO pressure (>8 kPa) gives the diameter selective growth of SWCNTs in the range of 1.5–1.7 nm.