Abstract A number of Raman-active peaks in the low-frequency region (100–600 cm −1) were observed for multiwalled carbon nanotubes (MWNTs). They were confirmed to be the radial breathing modes (RBMs) by using polarized Raman scattering. These RBMs originate from very thin innermost tubes (diameter, d=∼1 nm ) included in MWNTs, and the RBM peak of the smallest carbon nanotubes ( d=0.4 nm ) appears at 570 cm −1 . It has also been found that the innermost-diameter distribution calculated from the RBM frequencies agrees well with the observations of high-resolution transmission electron microscopy. This provides a new Raman-spectroscopy-based method for the determination of the innermost diameter of MWNTs.