The pore structure of supported organic-inorganic hybrid membranes was tested by modified gas permeation method. The results were discussed and compared with gas separation performances of the membrane. For the intermediate layer, the pore size is 225nm, which is in relatively good continent with the SEM observation. The pore size of SiO2/C6H5 organic-inorganic hybrid layer is 40-50nm, which is a little bigger compared with the gas separation performances. And for the SiO2/C6H5/MMA organic-inorganic hybrid layer, the pore is 24nm, which is close to the gas separation performances.