Nano-sized SiO x is synthesized by an evaporation and condensation process using induction melting with the injection of various mixed gases. In particular, the effects of O2 gas on their microstructures and electrochemical properties are investigated. The x-values of the synthesized SiO x can be controlled from 1.18 to 1.83 by varying the O2/Ar ratios in the injected gas. A micro-structural analysis reveals that mixture of nanoparticles and nanowires is formed and that the relative amount of nanowire decreases with an increase of the oxygen contents in the injected gas. When x is low, crystalline Si is formed, but this phase disappears when x exceeds 1.28. From the electrochemical analysis of the various SiO x powders, the Initial Columbic Efficiency (ICE) decreases with the increase in the x-values – largely due to the oxidation of Li-ions such as Li2O and Li4SiO4. Cycle stability, however, is improved with the increase of the x-values, indicating that strain and stress during insertion and extraction of Li-ions are properly released due to the oxide buffer around the Si as well as the nano-sized structure. When x = 1.18, a specific capacity of 660 mAh g−1 with a columbic efficiency of 99.8% at the 50th cycle is obtained.