The coarsening behavior of copper nanopowders (50-150 nm in size) during mechanical milling was investigated as a function of milling atmosphere and milling time. Pure copper powders were milled under argon and air atmospheres for up to 80 h and the crystal structure, microstructure, and composition of the milled powders were investigated. It was shown that on milling the powders for 80 h under air atmosphere, the particle size was in the range of 7 to 80 pm due to sphere-to-sphere and/or sphere-to-flake welding and from the aggregation of the copper particles during milling. On the other hand, the particle size was only 7 to 35 pm, when milled under argon atmosphere even after 80 h of milling. There was no difference either in the crystal structure or lattice parameter of the copper powder after milling in the two atmospheres. The powders got contaminated with oxygen; the oxygen content increased with increasing milling time, reaching up to 4.97 wt.% when milled in air for 80 h. The oxygen got incorporated into the copper powders to form oxide layers, which were broken down during milling and thus played an important role in determining the final particle size and microstructure of the powder. A viable explanation was provided to explain the differences in the microstructure and sizes of the powder particles milled under the argon and air atmospheres. (C) 2014 Elsevier B.V. All rights reserved.