The effects of various factors, i.e., shield gas, current density, wire polarity, and wire material, which can influence oxidation of ball surfaces were investigated. Good quality aluminum balls were found to be formed if optimum electric and shield gas conditions were employed; for example, current density 2.5 GA/m2wire polarity: cathode; shield gas: Ar+H2. Good quality aluminum balls of 1.7 to 2.5 times the wire diameter could be formed by varying discharge time and/or current density. It was found that they could be obtained independently of the wire material. The relation between morphologies of aluminum balls and the degree Of oxidation was also investigated in order to examine the mechanism for obtaining a good quality bail. A close correlation was found between ball morphologies, i.e., eccentricity, sphericity, and constriction, and the degree of oxidation. The morphologies were significantly improved as the oxide film thickness was reduced.