The field-emission (FE) of diamond surfaces with hydrogen defects in the subsurface is investigated by time-dependent density-functional theory calculation. It is found that FE currents from diamond surface with hydrogen atoms in the subsurface region are significantly enhanced because the defect levels induced by the hydrogen atoms in the energy band gap have small ionization energy. The shapes of energy distribution of FE current density vary largely depending on the type of hydrogen defects. The present results suggest that high emission current from diamond surfaces at low electric fields observed in experiments is ascribed to hydrogen defects in diamond subsurfaces. (C) 2004 Elsevier B.V. All rights reserved.