To investigate the diffusion behavior of Tl ions, glass was immersed in TlNO3–KNO3 molten salt at 753 K for various lengths of time. The Tl ions embedded in the glass were then reduced to metallic Tl nano-particles by hydrogen at 803 K for 1 h. The depth of the ion exchange layer down to about 140 µm in the glass was found to be proportional to a square root of the length of immersion time. It was suggested that the ion exchange process is controlled by the diffusion of Tl ions from the surface to the inside. Reduction of Tl ions to metallic Tl with the hydrogen gas occurred only at the surface down to 4 µm. The concentration profile of the metallic Tl is discussed by nucleation-nucleus growth and the diffusion of Tl ions from deeper areas.