The diffusion coefficients of Ce(III) and Gd(III) in LiCl-KCl eutectic melt were determined in the temperature range between 673 and 823 K by chronopotentiometry. A new method was devised to minimize the error in defining the surface area of the working electrode. The immersed depth of the working electrode was changed by stages, and the relation between the change in the square root of the transition time and that in the surface area of the working electrode was substituted into the Sand equation instead of their absolute values. Diffusion coefficients obtained were written in following equationslog D-Ce = -2.69(+/-0.145) - 1670(+/-108)/T (D-Ce:cm(2)/s)log D-Gd = -2.78(+/-0.128) - 1670(+/-94.3)/T (D-Gd:cm(3)/s)The activation energies for diffusion and the diffusion coefficients of lanthanide ions in LiCl-KCl are discussed in connection with their ionic radii and the stability of their complex ions.