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Copper nanopowders are prepared by pulsed wire discharge (PWD) using copper wires in nitrogen gas. From experimental results obtained in the present study and published literature, a relationship to predict the particle size of powders prepared by PWD is proposed. A theoretical plasma/vapor density (D th), which is the most important factor for controlling the particle size is defined as m P E c -1, where m, P, and E c are the weight of the wire, pressure of nitrogen gas, and charged energy in the capacitor, respectively. From high-speed photographs obtained during PWD, the relationship between the measured D exp and D th is shown by D exp∝D th 0.6. The relationship between the median particle diameter d 50 and D th is found to be d 50∝D exp∝(D th 0.6)0.4 by transmission electron microscopy observations. This empirical relationship is in agreement with that expected from the formation and growth of particles via Brownian coagulation of free molecules. From the above relationship, it is possible to predict and control the particle size of powders prepared by PWD.