The thermodynamic Saam-Cole approach including the molecule-surface interaction was applied to explain the critical pore width at which the adsorption hysteresis of Nz on regular mesoporous silica disappears. The difference of the critical thickness of physisorbed layers on the pore wall upon condensation and evaporation was associated with the pore wall roughness inherent to the atomic structures, showing that the critical pore width for the disappearance of the adsorption hysteresis is in the range of 3.6 to 3.8 nm.