Gas transport and thermodynamic properties of polymethylmethacrylate (PMMA) blended with polyvinylmethylether (PVME) containing diblock copolymer of styrene and methylmethacrylate (PS-b-PMMA) as a compatibilizer were studied. Phase separation temperatures of PVME blends with styrenic random copolymer containing various amounts of methyl methacrylate (NIMA) were determined to calculate the interaction energies of the binary pairs involved in this system using lattice fluid theory combined with the binary interaction model. The separated phase structure of the blend that is related to interaction energy was controlled with an interfacial agent, i.e. PS-b-PMMA. The average diameter of the dispersed particles in PMMA/PVME blend was decreased for the blends containing PS-b-PMMA from 0 to about 5 parts per hundred (phr) and then leveled off at a fixed value. Accordingly, oxygen and nitrogen gas permeability coefficients of the blend membrane increased with compatibilizer content, until it reached 5 phr and then leveled off. A trade-off between permeability and selectivity for blends containing various amount of compatibilizer was observed. The continuous path from top to bottom of membrane formed by reducing the dispersed particle size might cause an increase of gas permeability at the same blend composition.