A thin nanoporous silica membrane was prepared by the two-step sol-gel method. First, silica membrane was formed onto the alpha -alumina support as an intermediate layer with fibrous silica colloid. Then, silica nanoparticles were deposited on the first layer of silica membrane surface again by the sol-gel method. It is generally found that gelation process, as liquid evaporates during drying, begins with the formation of fractal aggregates. Since an aggregate having the higher fractal dimension has the more complex void space than the aggregate having the lower fractal dimension, the silica nanoparticle concentration in the coating solution for the second step modification was selected low enough to avoid the aggregates with the higher fractal dimension. Thus, the prepared membrane had a dense skin layer with a nanospace as interparticle void space, which was proved by the AFM surface image analysis and the result from nitrogen adsorption porosimetry.