On the basis of the atomistic simulations of electrowetting of single-walled carbon nanotubes by mercury, the electrowettings in double-walled carbon nanotubes are studied. Classical molecular dynamics simulations in conjunction with a macroscopic electrocapillarity model are carried out to clarify the effect of inner tube’s size on electrowetting. Results show that there is a great difference between electrowetting in single-walled and double-walled carbon nanotubes. The inner tube plays a key role in the electrowetting of double-walled carbon nanotube. Very different phenomenon occurs when the size of the inner tube is changed.