Si and SiOx nanoparticle-embedded nanoporous carbons were successfully synthesized by reducing a SiO2 opal-carbon composite precursor with Mg, and their interstitial nanospace in carbon pores was tunable by partly etching the SiO2 particles in the precursor. It was revealed that dispersive loading of Si and SiOx nanoparticles in carbon nanospace is necessary to enhance the charge-discharge performance in addition to providing the interstitial nanospace as a buffer space for the volume change associated with Si-Li reactions.