Abstract: In the present study, the dependence of dispersivity on the thickness of aquifer has been investigated. The physical model used in the study consisted of a rectangular Plexiglas tank with inner dimensions of 720 mm length, 100 mm width and 1200 mm height. Sodium chloride with an electrical conductivity (EC) of 14 dS m-1 was selected as conservative contaminant. Porous media used in the experiments consisted of homogeneous coarse and medium sand particles. The experiments were performed in two different stages. In the first stage, 10 experiments in the aquifer with coarse sand particles and eight experiments in the aquifer with medium sand particles with constant thickness of 100 mm and flow velocities ranging from 4.5x10-5 to 11.25x10-5 m sec-1 were performed. In the second stage, experiments with thicknesses of 200-1000 and 100 mm layer distance were performed. The flow velocity in the second stage was maintained at 9.0x10-5 m sec-1 for each simulated aquifer, based on the previous studies. Results of the study indicated that: (1) the dispersivity values obtained for coarse and medium sand particles with 100 mm thickness were in the range of 0.25-0.65 and 0.11-0.33 cm, respectively and the mean values of dispersivity for both aquifers were in the range of 0.01 to 1.0 cm which are in agreement with the findings of other researchers, (2) the dispersivity values obtained for aquifers with coarse and medium sand particles and thicknesses of 200-1000 mm were in the range of 0.31-0.63 and 0.14-0.46 cm, respectively, which are in agreement with the findings of others as well, (3) the dispersivity of sandy porous media is independent of particle size and (4) in homogeneous sandy aquifers with coarse and medium particle size, the dispersivity is independent of aquifer thickness.