C-60-filled single-walled carbon nanotubes (SWNTs), so-called fullerene peapods, have been successfully doped with iodine. High-resolution transmission electron microscope observation revealed that the iodine atoms could be doped in the intratubular position. The doped intratubular iodine atom(s) can assist the coalescence of the C-60 molecules inside SWNTs. Heating the iodine-doped peapod at 550 degrees C induces the transformation of C-60 molecules into a tubular structure. The doped iodine atoms can be intercalated between the outer host SWNT and the newly formed inner SWNT.