The objective of this study is to determine bow breakup and monosized droplet generation of a molten metal (tin-lead, Sn63-Pb37) jet is affected by oxygen concentration. The major experimental equipment is the monosized droplet generator, which breaks up a laminar stream of liquid metal jet to produce monodispersed droplets by forced vibration generated from a piezoelectric disk within the proper frequency range. The results show that high oxygen concentration retards the breakup process of the molten metal jet, and the "critical oxygen concentration" is about 910 ppm at 1 atm and 350degreesC, which will result in sudden breakup failure. The molten metal jet can be further divided into three regimes, according to the effect of oxygen concentration, i.e., a "breakup regime," an "incomplete breakup regime," and a "breakup failure regime. "As long as a monosized droplet stream is formed, the breakup lengths of jets with different oxygen concentrations are almost the same, even when oxide islands appear. The behavior of the excited liquid metal jet still conforms to Rayleigh theory, i.e., lambda(min) = piD, and the working frequencies are limited within the range piD < lambda < 2lambda(opt), both when surface tension is up to 500 X 10(-3) N/m, and when oxide islands appear, i.e., in both the breakup and incomplete breakup regimes. However, in the breakup failure regime, Rayleighs theory is not applicable, and even Wallace's surface tension reduction theory [12] cannot explain this reaction. It must be explained by integrating the combined research models of Haj-Hariri and Poulikakos [13] and Artem'ev and Kochetov [14], i.e., the oxide film islands grow and join until reaching high surface flexural rigidity at the critical oxygen concentration, which will suppress the capillary instability and result in a sudden failure of the breakup process.