Monolithic carbon/nano-silicon composites with a three-dimensionally ordered macroporous (3DOM) structure were synthesized via a nanocasting route using a macro- and mesoporous silica monolith as a hard template for carbon with similar hierarchical porosity, followed by chemical vapor deposition to infiltrate mesopores with silicon nanoparticles. Diiodosilane was used as the silicon precursor to produce nano-silicon upon thermal decomposition. X-ray photoelectron spectroscopy revealed the presence of both elemental silicon and oxidized silicon in the porous carbon. Silicon was dispersed uniformly inside 3DOM carbon without forming large agglomerates. The as-synthesized material was X-ray amorphous. A lithiation experiment showed an initial charge capacity of 920 mAh g-1 and a reversible Li+ capacity of 332 mAh g-1 for the carbon/nano-silicon composite. The lower-than-expected capacity is attributed to partial oxidation of nanosized silicon in the composite structure. A mechanism for decomposition of diiodosilane in this system is proposed.