Flow through diffusion method was used to determine the effective diffusion coefficient and the appropriate parameters for modeling iodide ion diffused through the compacted clay materials of kaolinite and bentonite which were supplied locally and from USA. The effects of clay structural characteristics on diffusion. the mechanism of iodide ion diffusion through the compacted bentonite were studied.The effective diffusivity for kaolinite decreases with increasing bulk density and concentration of the diffusion species (/-). The iodide ion was strongly repelled from the bentonite particle surface, the predominant diffusion path is in the interstitial water. The relationship between the iodide flux (}) and the formation factor (F) for the diffusion of iodide ion through the compacted clays can be described by the following empirical equation:where A and B were constants. The inverse of iodide diffusion flux decreases with increasing formation factor. The types of clay and their structures have a significant effect on the diffusion of iodide ion through compacted clays.