Flow-through capacitor (FTC) with carbon nanotube (CNT) electrode was employed to remove NaCl from saltwater solution. According to the investigation by transmission electron microscopy (TEM) and nitrogen adsorption, the BET specific surface area of CNT electrodes decreased with increasing nanotube diameter, which had a narrow distribution in each sample. Besides, removal of NaCl per unit weight of CNT electrode increased linearly with the BET specific surface area of CNT electrodes. As a result, CNT electrode with the smallest nanotube diameter had the best removal characteristic, with removal efficiency up to 95% and energy-consumption of 3.0 Wh L-1, and the process of regeneration could be carried out easily in a short time. Comparing removal experiment with charge/discharge test, it was concluded that FTC based on the same electro-adsorption theory as double layer capacitors. The cyclic voltammograms of CNT electrode in salt water showed that the current occurred by adsorption and desorption of ions in the range of -0.3-0.9 V, which directly explained the reason why CNT electrode could be used to remove NaCl.