Gas permeability of high polymer is important as a physical property in the gas barrier evaluation of vehicle tires and some fuel hoses from the viewpoint of energy-loss. The precise measurement of permeability rate of oxygen and steam is recently required in the field of organic EL display and fuel cell.This review describes the well-known mechanism of the gas permeability in the elastomers and the classification of gas transmission rate measuring methods as standard testing methods. The relationships between gas permeability and manufacturing process factors are also introduced.By using L18 orthogonal array, the results allow not only specifying effective factors and levels but also optimizing those for gas barrier property. Among the eight factors examined, primary factor of gas permeability is found to be the type of polymer, followed by the shape of inorganic filler, while the crosslink density of vulcanized rubber is not so influential.