Nanoparticles of copper have been synthesized by wire explosion technique in different ambient chamber pressure in nitrogen environment and effect of the ambient chamber pressure was studied. A copper wire of diameter of 125 pm and 6.1 cm in length is exploded at 1 bar, 500 mbar, 100 mbar and 50 mbar ambient chamber pressures. Current density in the order of 10(6) A cm(-2) from a 1.85 mu F capacitor charged up to 10 kV has been applied to disintegrate the wire. The particle sizes were calculated from the peak of the X-ray diffraction pattern as well as from the transmission electron microscope image. The average particle size is found to be smallest for 500 mbar. Increase in average particle size is observed either with increase in the ambient chamber pressure to 1 bar or when it is decreased to 100 mbar, which makes the intermediate pressure of 500 mbar optimum for synthesis of smaller size particles. In-situ optical emission spectroscopy is carried out to study the plasma chemistry of the process and to determine the plasma parameters during the explosion. The mechanism of arc plasma formation is also studied. It is observed that the mechanism of the arc plasma formation plays a significant role in wire explosion process, which is found to be responsible for increasing particle size with either increased or decreased ambient chamber pressure from the intermediate pressure of 500 mbar. (C) 2012 Elsevier B.V. All rights reserved.