Sn nano powder has great potential for the applications of various functional materials such as solder paste, ITO, ATO, anode materials for fuel cell, and gas sensor. In this paper we introduce on our new development for fabricating high purity Sn nano powder by low temperature phase transformation. From the practical point of view, this process is more advantageous than the previous methods for producing Sn nano powder. Sn ingot material having a purity of > 99.99% was stored at -85oC to be transformed from beta-Sn (tetragonal, 0.136 cm3/g) to alpha-Sn(diamond cubic, 0.174 cm3/g). A large discrepancy of lattice volume in these two phases enlarges stress concentration during phase transformation, followed by forming a number of fine structures less than 100 nm in width on the ingot surface. By cyclic transformation and subsequent sono-milling, pure Sn nano powder (~50 nm) having the same purity as the starting material could be fabricated. The formation mechanism of Sn nano powder in this study is discussed in terms of microstructural development and phase transformation.