In this work, nanostructured, monodisperse Si spheres have been prepared by a facile, controllable magnesiothermic reduction method using monodisperse SiO2 spheres as the precursor. The reaction conditions for magnesiothermic reduction have been optimized, yielding desired product and byproduct. Carbon coating on Si nanocrystals is performed by an easy chemical vapor deposition (CVD) method. The effect of carbon coating on electrochemical performance of Si is investigated. The results show that the original monodisperse spheric morphology of SiO2 can be preserved during magnesiothermic reduction and the cycling stability of Si can be enhanced by carbon coating. This work provides a facile, scalable method to synthesize nanostructured Si and Si/C composites.