Liquid crystalline polysiloxane with laterally attached side chain has been synthesized to facilitate the study of relationships between the microstructure of polymer and the gas transport properties (permeability and selectivity). The gas permeability and selectivity of the membrane to CO2, O-2, N-2, and CH4 have been measured at various temperatures from 3 to 38 degrees C and at 76 cmHg. The permeability of the polymer to different gases decreased in the order, P-CO2 > P-O2 > P-CH4 > P-N2, as shown in glassy polymers. Activation energies for the gas transport were somewhat larger than those obtained for other polymers. The gas selectivity, (P-O2/P-N2) and (P-CO2/P-CH4), of the membrane strongly depended on the temperature and the maximum values of the selectivities were showed at the glass transition temperature. The gas selectivity of the polymer is mainly controlled by the solubility selectivity, as is the case for other rubbery polymers. The effect of microstructure of liquid crystalline polysiloxane with laterally attached side chain on the gas transport properties is discussed.