To analyze the influence of environmental media on nanopowders prepared by electrical explosion method, electrical explosion experiment is carried out using copper wire under water and different air pressures. Voltage and current waveforms are measured by Rogowski coil and high-voltage probe. The basic process of electrical explosion and the function of environmental media in the course of electrical explosion are analyzed by combined characters of the voltage, current, and energy deposition. The particle size of explosive products is analyzed by transmission electron microscopy. Results show that it is mainly after the vaporization stage of the copper wires that the medium may affect the formation of the products. Effects of the medium on the electric explosion of metal wires include the restriction of the medium on the expansion of the metal vapor, the influence of the medium ionization on breakdown of the copper surface as well as the cooling of the copper vapor and the plasma. For the water medium, the diameter of the products distributes in a wide range but is mainly in the range about 10 to 20 nm, while in the air medium, the average particle size is about 40 nm, basically ranging from 20 to 100 nm.