Nano-composites of tin and tin dioxide particles were synthesized on carbon nanotubes by the single-atoms-to-clusters (SAC) method, and their structures were investigated by high-resolution transmission electron microscopy. By changing the heat-treatment temperature during the SAC process, two different types of samples were obtained. The samples prepared around 450K were aggregates of 2–4nm-sized tin dioxide nanoparticles, and their size distributions on carbon nanotubes are in the range 20–40nm. The other samples formed above 600K had a core–shell structure of diameter 20–40nm. The core and shell were made of tin single crystal and disordered oxidized tin, respectively. The thickness of the oxidized layers was ca. 4nm.