The separation performances of divalent ions (Ca2+ and Mg2+) by electrodialysis (ED) were compared using a constant voltage mode of operation. For the batch tests, TS-1-10 ED equipment was employed. During the experimental studies, the effects of electrical potential applied and flow rate of streams on the separation performance of each cation have been monitored by measuring the outlet concentrations of the dilute stream. The operation time was short when a high potential was applied. On the other hand, the effect of flow rate on separation performance was not so apparent. Specific power consumption increased with an increase in potential applied, although it was not sensitive to the change in flow rate.