Molecular dynamics simulations were performed to study the hydration of Na+ inside carbon nanotubes (CNTs) under external electric fields from 0 to 0.1 V/nm. The simulation results show that the hydration of Na+ varies with the electric field intensity (E). The coordination number in the first hydration shell of Na+. increases with the increase of E. The analysis of orientation distributions for water dipole shows the Na+ hydration is strengthened when the direction of external field is aligned with the electric field generated by Na+, and is weakened when these are opposite. When E is aligned with the electric field of Na+, the strength of hydration is non-monotonic as E and reaches its maximum when E is equal to 0.01 V/nm. This finding provides some guidance for the application of electric field in ion-transporting channels. (C) 2013 Elsevier BY. All rights reserved.