Synthesis behavior and microstructural characteristics of nano-sized SnO2 powders were investigated. SnO2 powders were synthesized by controlled heat treatment of nano-sized Sn powders in air. Microstructural observation revealed that the nano-sized SnO2 powders could be successfully prepared by controlled oxidation treatment with a low heating rate and further ball milling. The synthesis behavior was analyzed based on the observed microstructural characteristics dependent on heating rate and milling condition.