We have prepared silica composite membranes with different silica particle sizes in order to determine the transition region between viscous flow and Knudsen diffusion. Silica sols with particle sizes of 70, 100, 300, and 500 nm prepared by the sol-gel method were used to synthesize four types of the silica composite membranes supported on porous stainless steel disks by the DRFF and SRFF method. For the membrane prepared using < 100-nm silica particles, viscous-flow-dominated permeation behavior was found, as the silica layer could not form on the support because of the loss of small silica particles through the large macropores of the stainless steel substrate. In the case of the silica composite membranes fabricated with 300- and 500-nm colloidal silica sols, even though crack-free silica layers formed, they showed viscous-flow-dominated permeation behavior, which can be attributed to large interstitial voids among the colloidal silica particles. In contrast, for the silica composite membranes fabricated with 100-nm colloidal silica, a crack-free silica layer was successfully synthesized, and the H-2/N-2 permselectivity was 3.4-3.7, which approaches the theoretical H-2/N-2 selectivity for the Knudsen diffusion mechanism (3.74). Therefore, it can be concluded that the most suitable size of silica particles is about 100 nm for the modification of porous stainless steel substrates by means of the DRFF and SRFF method.