Oxidation behavior and microstructural characteristics of nano-sized Sn powder were studied. DTA-TG analysis showed that the Sn powder exhibited an endothermic peak at 227°C and exothermic peak at 560°C with an increase in weight. Based on the phase diagram consideration of Sn-O system and XRD analysis, it was interpreted that the first peak was for the melting of Sn powder and the second peak resulted from the formation of SnO_2 phase. Microstructural observation revealed that the SnO_2 powder, heated to 1000°C under air atmosphere, consisted of agglomerates with large particle size due to the melting of Sn powder during heat treatment. Finally, fine SnO_2 powders with an average size of 50nm can be fabricated by controlled heat treatment and ultrasonic milling process.