The effect of synthetic leachate on the hydraulic conductivity of a clayey soil in the Urmia city landfill site, Iran, was investigated using a triaxial permeability apparatus. The bladder accumulators were fabricated for flexible wall triaxial permeability apparatus to facilitate synthetic leachate permeation in the apparatus. The landfill soil was tested and classified as silty clayey sand (SM-SC) and did not fulfill the requirements as a soil for landfill liner. Hence, the landfill soil was mixed with 12% of a clayey soil from a nearby location to bring the index parameters of the mixed soil into a range specified for a landfill liner soil. The synthetic leachate was prepared with three different Ca++ concentrations. The mixed soil was compacted wet of optimum water content and permeated with tap water. After equilibrium, the tap water was replaced with synthetic leachate with three different Ca++ concentrations. The hydraulic conductivities of 1.33×10-8 cm/s, 1.81×10-8 cm/s, and 1.77×10-8 cm/s were obtained for the soil permeated with synthetic leachate, compared with the average hydraulic conductivity of 1.52×10-8 cm/s when permeated with tap water for tests A, B, and C, respectively. The synthetic leachate with a 1000 mg/L Ca++concentration caused about a 13% reduction in hydraulic conductivity in test A which could be due to the decrease in the void ratio because of the consolidation of the sample. The percentages of increase of hydraulic conductivity for tests B and C, in comparison with the values for tap water permeation, were 18% and 20%, respectively, which is attributed to double layer contraction and increased pore space, resulting from the adsorption of divalent cations into the soil matrix.