The composition, structure, light emission and oxidation kinetics of Si1-xGex alloy nanocrystals (∼3nm in diameter) synthesized by nonthermal plasma have been investigated. Itis found that the synthesized nanocrystals are neither a mixture of Si nanocrystals and Ge nanocrystals nor Si-Ge (Ge-Si) core-shell nanocrystals. The H coverage at the surface of Si1-xGex nanocrystals decreases with the increase of the Ge atomic fraction. The incorporation of Ge enhances the oxidation of Si1-xGex nanocrystals when the atomic fraction of Ge is <0.5. No shift in photoluminescence from hydrosilylated Si1-xGex nanocrystals is observed when the atomic fraction of Ge varies between 0 and 0.1, indicating that the bandgap of Si nanocrystals is similar to that of Ge nanocrystals at a nanocrystal size of 3nm.