The transition boundary of the temperature dependence of small gas permeability through molecular sieving membranes was investigated by the use of grand canonical ensemble molecular dynamics (GCMD). The temperature dependence of H-2, Ne, Ar, O-2, N-2, and CO2 permeability through molecular sieving membranes was examined by use of membrane models having cylindrical pores with a range of 0.3-0.5 nm. Our simulation successfully demonstrated that the observed permeability varied from nonactivated transport to activated transport with increasing temperature as the pore size of the membrane models approached the molecular diameter of the permeating species. This suggests that the permeation mechanism changed to molecular sieving. Activated transport was found when the pore size of the membrane became smaller than 1.2 times that of the molecular diameter of the permeating species. There was no clear correlation observed between the static interaction between the gas and pore wall and the transition boundary of the temperature dependence of permeability. The observed activation energy for permeation was in the range of 1-7 kJ/mol, dependent on the permeating species.