Size tuning of metallic copper fine particles is highly important for their application to conductive pastes. Uniformly sized metallic copper fine particles could be successfully obtained by a chemical reduction of CuO micro powders using hydrazine in the presence of polypeptides as the stabilizing regent. Easy tuning of their particle sizes was successfully achieved via controlling the molecular weight of polypeptides. The obtained particles were covered by polypeptide capsules which were approximately 2 wt% of total mass. They are highly stable even under ambient conditions. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.