Double-wall carbon nanotubes (DWNTs) were prepared in quantitative amount from the chains of C-60 molecules generated inside single-wall carbon nanotubes (SWNTs), The C-60 molecules inside the SWNT unchange up to similar to 800 degreesC without dedoping. Further heating at similar to 1200 degreesC induces the coalescence between C-60 and eventually the Coo molecules transform into a tubular structure. The DWNTs thus prepared show radial breathing mode (RBM) Raman scattering associated with inner tubes as well as those from parent outer tubes, which are all explained by a model of diameter pairing in DWNT with an inner tube diameter smaller by 0.71 +/- 0.05 nm than an outer one.