The solid-state reactive diffusion between Ni and Sn was experimentally studied using Sn/Ni/Sn diffusion couples. The diffusion couples were prepared by a diffusion bonding technique and then annealed at temperatures between T = 433 and 473 K for various times in an oil bath with silicone oil. At the Ni/Sn interface in the annealed diffusion couple, a rather wavy layer of Ni 3 Sn 4 was observed at T = 453 and 473 K, but granular particles of Ni 3 Sn 4 were recognized at T = 433 K. The average thickness l of the Ni 3 Sn 4 layer monotonically increases with increasing annealing time t according to the equation l = k(t/t 0 ) n , where t 0 is unit time, 1 s. The experimental results provide n = 0.46 and 0.41 at T = 453 and 473 K, respectively. The values of n smaller than 0.5 indicate that the grain boundary diffusion contributes to the rate-controlling process and the grain growth occurs at certain rates. Under the present annealing conditions, the thickness of the Ni 3 Sn 4 layer is more than one order of magnitude smaller than the total thickness of the AuSn 4 , AuSn 2 and AuSn layers formed by the reactive diffusion between Au and Sn. This indicates that the Ni layer is an effective inhibitor against the reactive diffusion between the Sn-base solder and Au/Ni/Cu multilayer conductor alloys during annealing at solid-state temperatures after complete dissolution of the Au layer into the molten solder.