Ultrafine SnO2 nanoparticles were successfully synthesized by microwave heating. The SnO2 nanoparticles worked as a rechargeable electrode material for a lithium-ion battery. Micro-Raman spectroscopy, X-ray diffraction, and cyclic voltammetry studies of the SnO2 nanoparticles elucidated that the lithium insertion-deinsertion reactions during the 1st cycle were caused by both redox of SnO2 and Li-Sn alloying-dealloying reactions. The redox reactions were irreversible and disappeared via charge-discharge cycling, and only the Li-Sn alloying-dealloying reactions took place at the later cycle.