Diameter distributions of single-wall carbon nanotubes (;SWNTs) produced by an are discharge with a Rh-Pt catalyst at various helium pressures from 50 to 1520 Torr were studied by transmission electron microscopy and Raman scattering spectroscopy. The distribution shifted systematically to small diameter with the decrease of helium pressure. In addition to the helium pressure, the diameters of SWNTs were dependent on the locations inside the are evaporator where SWNTs were collected (viz., either on the chamber walls or around the cathode). The most frequently occurring diameters could be controlled from 0.95 nm at 50 Torr to 1.4 nm at 1520 Torr. The effect of helium pressure on the diameters of SWNTs is discussed in connection with the corresponding change in the yield of higher fullerenes.