Porous silicons (PSs) were prepared from a lightly doped (boron; 7 × 1014/cm3) crystal silicon (c-Si) wafer by anodization in an aqueous hydrofluoric acid (HF) solution of different concentrations (25−55%). The porosity of these PSs was examined by nitrogen adsorption at 77 K. The relationship between the pore structure and the HF concentration upon anodization was examined. All the PSs investigated showed the type IV isotherm with a type H2 desorption hysteresis, indicating mesoporosity. The pore size distribution was derived from nitrogen adsorption isotherms with the BJH method. The total pore volume of the PSs decreased gradually with increasing HF concentration, but the specific surface area increased with concentration above 37 wt %. The pore size distribution varied with the HF concentration. The profile of the distribution became sharper and shifted to a smaller size with the increase in the HF concentration.