Pinhole-free Pd/Ni alloy composite membranes with a diffusion barrier have been fabricated on mesoporous stainless steel supports by vacuum electrodeposition. To improve the structural stability of Pd alloy/Ni-316-L stainless steel (SUS) composite membranes, a thin intermediate layer of TiN by a sputtering method was introduced as a diffusion barrier between the Pd/Ni active layer and a modified SUS substrate. The composition and phase structures of the alloy film were studied by energy-dispersive spectroscopic analysis and X-ray diffractometry; the typical Pd/Ni plating had a composition of 82% Pd and 18% Ni. The electron probe microanalyzer profiling analysis indicated that the improved membranes were structually stable. The Pd/Ni alloy composite membrane with a diffusion barrier of TiN yielded a high separation performance for hydrogen. The composite membrane with a diffusion barrier of TiN shows very good stability under the mixture gas of 50% H-2/50% N-2 for an operation time of more than 60 days. The purity of hydrogen can be raised from around 99.7 to 99.9% or above.