We have investigated the half-metallicity in hydrogenated carbon nanotubes under an electric field as a function of the electric field direction by using the density functional theory calculation. We found that the electric field required for the half-metallicity can be significantly reduced by controlling the field direction and the reduction rate increases with the nanotube diameter. The field direction effect can be understood in a simple model based on the electrostatic potential difference between the spatially-separated edges. (C) 2013 Elsevier B. V. All rights reserved.