Through the volume change of Sn in a low-temperature phase transformation, the Sn nanopowder with high purity was fabricated by an economic and eco-friendly process. The fine cracks were spontaneously generated in Sn ingot, which was reduced to powders in the repetition of phase transformation. The Sn nanopowder with 50 nm in size was obtained by the 24th repetitions of phase transformation by low-temperature and ultrasonic treatments. Also, the SnO2 powder was fabricated by the oxidation of the produced Sn powder to the ingot and milled by the ultrasonic milling method. The SnO2 nanopowder of 20 nm in size was fabricated after the milling for 180 h.