Abstract The interdiffusion between niobium/permalloy bilayer thin films annealed at 100–450 °C is studied. Ni markedly diffuses from the permalloy layer into the Nb layer. Consequently, the electrical resistance of Nb/permalloy bilayer thin films and coercivity of the permalloy layers increase with increasing annealing temperature. The interdiffusion of Ni and Nb increases with decreasing film thickness. This is due to short-circuit diffusion along the grain boundaries, because the thin metal layers consist of small grains. The apparent activation energies of the interdiffusion are 0.74 eV for Nb (50 nm)/NiFe(80 nm) bilayer thin films, 0.64 eV for Nb (30 nm)/NiFe (20 nm), and 0.50 eV for Nb (10 nm)/NiFe (12 nm) bilayer thin films.