Porous silicon powder has been prepared for the first time by a hydrothermal technique. Strong red-emission around 680 nm was observed in the samples. Transmission electron microscopy studies reveal that the as-prepared powder was composed of plate-like particles, which were found to be porous structures of approximate to 2-3 nm primary units. These results reveal that the particles of porous silicon powder have a microstructure and luminescent properties similar to those of anodic-etched silicon films.