Lithiation and delithiation of porous silicon were studied using reflection high energy electron diffraction (RHEED), two-dimensional photoelectron diffraction, and a stereo atom-scope, which is realized by the combination of a display-type spherical mirror analyzer and circularly polarized soft X-ray. A nanosized porous silicon layer was prepared by electrochemical etching of p-type silicon (001) wafer in ethanolic solutions containing hydrofluoric acid. The morphology of the as-grown porous silicon as observed using SEM was filled with about 9 nm holes. This porous silicon also retains the crystallographic orientation of the wafer from which it was etched and is optically active with visible photoluminescence. The measured RHEED pattern and 2 pi steradian Si 2p photoelectron diffraction pattern from Si (001) surface showed an increase in lattice constant by lithiation, and that change in lattice constant was restored to its original values by delithiation.