We present an efficient method to extract inner shells of double-wall carbon nanotubes (DWCNTs) in liquid phase. The extraction of inner from outer shells is achieved by cutting the DWCNTs with vigorous sonication in water containing surfactants. The extracted shells are perfectly isolated single-wall carbon nanotubes (SWCNTs) and can be separated using density gradient ultracentrifugation. Statistical analysis using high-resolution transmission electron microscopy reveals that the enrichment of SWCNTs with narrow diameter (0.62-1.0 nm) up to 100% is achieved from highly pure DWCNTs. Furthermore, the (5,4) SWCNTs, which have the diameter of 0.62 nm, are concentrated. Our findings provide a novel way to obtain very narrow, highly isolated SWCNTs with ultraclean surface that have not been obtained in conventional synthesis methods.