Nanophase tin powder having sizes ranging from 6 to 40 nm was synthesized by the inert gas condensation method using helium as the convection gas. As-synthesized particles smaller than 8 nm were the amorphous tin oxide. As-synthesized particles larger than 10 nm can be characterized by the core-shell structure comprising inner crystalline tin core and outer amorphous tin oxide shell having a thickness of about 4 nm. Upon annealing in air, the oxidation of nanophase tin particles strongly depended on particle size. With increasing particle size, the transformation into the crystalline phases took places at a higher temperature. Calculation of the size dependent melting temperature of tin particles indicates that melting of the tin encapsulated with the amorphous tin oxide took place prior to the oxidation.