Abstract: This invention relates to a metallic composite membrane for separating hydrogen from a mixed gas, a membrane module and a manufacturing method thereof. The composite membrane consists of a three-layer sandwich structure, e.g. the porous metal substrate, an intermediate layer and the hydrogen-selective dense thin metal layer. In the embodiments of the invention, the porous metal substrates were first pretreated to reduce their surface roughness without reducing their gas permeability. The pretreated substrates were coated with intermediate layer, wherein the intermediate layer served as not only a barrier layer to prevent interdiffusion between the substrate and the hydrogen-selective layer, but also a surface modifier to reduce surface roughness and pore size of the substrate. A hydrogen-selective metal layer was then deposited on the intermediate layer-coated substrate by coating methods.