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 $227^{circ}C$ and exothermic peak at $560^{circ}C$ 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 $1000^{circ}C$ under air atmosphere, consisted of agglomerates with large particle size due to the melting of Sn powder during heat treatment. Finally, fine SnO2 powders with an average size of 50nm can be fabricated by controlled heat treatment and ultrasonic milling process.