Microstructure and hydrogen permeability of Nb-Ni-Ti alloy membranes for hydrogen separation were investigated. XRD and SEM were employed to characterize the phases and microstructure of Nb-Ni-Ti ternary alloys; hydrogen diffusion coefficients of these membranes with different thickness, composition or microstructure, were measured in Devanathan-Stachurski dual-cell system. The results indicate that hydrogen diffusion coefficient increases with the increasing thickness of these membranes. Basically these alloys are all of two-phase, i.e. the primary bcc-Nb(Ni,Ti) solid solution phase and the eutectic phase of bcc-Nb(Ni,Ti)+β2-NiTi, while just a small amount of NiTi2 phase is observed in very few samples. Microstructure and composition play an important role on hydrogen diffusion. Hydrogen diffusion coefficient is increased with the increase of the content of solid solution phase; the addition of Ni and Ti lead to the variety of eutectic phase composition; the presence of NiTi phase and NiTi2 phase decreases the hydrogen diffusion coefficient, but benefits the resistance to hydrogen embrittlement induced by the crystal expansion/contraction during atomic hydrogen permeation/release. Hydrogen diffusion coefficient of Nb-Ni-Ti alloy membranes falls in the magnitude of 10-9 m2/s.