Molecular simulation of permeation by various single gases (H-2, He, Ne, Ar, O-2, N-2, CO2, CH4) through different density amorphous silica membrane models at several temperatures has been carried out using the grand canonical ensemble molecular dynamics technique. The purpose of this study is to investigate the gas permeation mechanism through amorphous silica membranes from a microscopic point of view with regard to the microscopic structure of the membrane. The calculated gas permeability in two membrane models, with density values of 1.25 g/cm(3) and 1.50 g/cm(3), is found to decrease with increase in molecular weight of the permeating species, except for He permeation in a 1.50 g/cm(3) density model that showed higher permeability than that of H-2. This means that molecular sieving occurs in this system, which agrees with experimental data in chemical vapor deposition (CVD) silica membranes. Molecular trajectory analysis also supports the presence of molecular sieving that clearly reveals He has many permeation paths in the membrane in addition to the path of H-2. The permeability for all gases decreases as the temperature increases, which is inconsistent with the experiments. This inconsistency would be due to the relatively broad pore size distribution of the model used in simulations and we concluded that activated permeation against temperature needs the pore size less than 0.5 nm with sharp distribution.