Copper fine particles were successfully prepared by hydrazine reduction of cupric oxide (CuO). Changes in the temperature and pH values during the redox reaction of cupric oxide and hydrazine monohydrate were observed in detail. These changes reflected the reaction steps leading to the formation of metallic copper particles. The reduction of cupric oxide proceeded mainly during the part of the process where the pH value changes little or only very slowly. This phenomenon strongly indicates that the reaction and the particle structures can be established by control of the pH during the formation.