Neutron radiography (NR) using cold neutron beam was applied to the tracer diffusion coefficient measurement of lithium ion in Li1.33Ti1.67O4. The diffusion couples with different lithium isotope concentrations were annealed at 860 to 900 degrees C and the isotope profiles of lithium ion in the sample were measured by NR. The diffusion profiles obtained for the samples annealed at higher temperatures show good agreement with Fick's law, whereas the diffusion was not carried out well at the interface for the lower-temperature-annealed samples. The values of diffusion coefficients obtained from the profile fitting were a little smaller than those expected from the electric conductivity. The NR method was found to be the most useful method to measure the tracer diffusion coefficient of lithium ion in solids. (C) 1999 Elsevier Science B.V. All rights reserved.