A study was conducted to demonstrate highly reversible and large lithium storage in mesoporous silicon-carbon (Si/C) nanocomposite anodes with silicon nanoparticles embedded in a carbon framework. A magnesiothermic reduction approach was designed to synthesize mesoporous Si/C nanocomposites with ultrasmall, uniform silicon nanoparticles embedded in a rigid mesoporous carbon framework. The mesoporous Si/C nanocomposites had a high surface area of up to 1290 m2 g-1 and a bimodal pore size distribution. The small pores (ca. 2 nm) inside the carbon framework generated by magnesiothermic reduction could prevent the uniform small silicon nanoparticles from shedding, provide sufficient void space to accommodate the large volume expansion of Si during Li insertion, and facilitate Li+ diffusion between the electrolyte and Si nanoparticles.