Palladium membranes were prepared on an a-alumina support by metal-organic compound chemical vapor deposition (MOCVD) method from palladium(II) acetate precursor. Permeation properties of hydrogen and helium gas were studied as a function of the number of times of deposition of palladium on the peeling off phenomenon of palladium, which is common in electroless plated membrane, was observed. Silica was introduced into the pores to prevent the palladium grain from peeling off. The palladium-silica conjugated membrane does not show the peeling off phenomenon and can withstand the high temperature up to 800 degreesC which is the upper limit of our apparatus. The separation factor for hydrogen gas over carbon dioxide gas was improved with the increase of number of times of silica coating by sacrificing the H-2 permeation and finally increased to four times. The improvement on the separation of hydrogen gas over carbon dioxide for palladium-silica conjugated membrane was evaluated and a model of permeation pattern (palladium and silica) was proposed. This model suggests that the separation factor for hydrogen over carbon dioxide could be improved by introducing silica layer because the silica layer fills the pores and reduces the gas permeation without sacrificing the hydrogen permeation through the palladium region. These results indicate that the introduction of silica into the palladium grain is a promising means to improve the hydrogen separation performance of palladium based composite membranes.