Electrokinetic remediation is one of the most promising in situ or ex situ processes for decontamination of heavy metals, anions and polar organic compositions from the soil. In this study, we investigated the efficacy of this method in removing mercury from clayey soil (with kaolin to sand weight ratio of 2:1) and from gold mine tailing dam sediment and determined optimum removal conditions. Total mercury concentration in the soil and sediment samples was 800 and210 mg / kg, respectively, and duration of the experiments was considered as 32 and 30 days, respectively. The experiments were conducted on the samples with two voltage gradients (1.0 and 1.5 VDC/cm) to assess the effect of voltage gradient when using 0.1 M Na-EDTA, 0.1 and 0.4 M KI solutions and distilled water. The results showed that the best efficiency (99.07%) was reached when the 0.4M KI concentration was used with the 1.0 VDC/cm voltage gradients, leaving a residual concentration of 7.47 mg/kg in the soil after the treatment. In addition, the best efficiency for mercury removal from the tailing dam sediment was achieved in these conditions. The electrical current profiles, pH of the anode and cathode reservoirs, electrical conductivity of the soil and its moisture were then plotted and discussed.