By means of ab initio density functional theory calculations, we investigated the influence of the surface reconstructions on the geometries, stability, electronic structures and the electron affinity of the hydrogenated nanodiamonds, which is achieved by the dehydrogenation from neighboring hydrogenated sites. Our results show that the modifications of the geometries can tune the HOMO-LUMO gaps. The spatial variation of the LUMOs depends rather on the C-H bond length than on the respective surface sites and the causes are analyzed. For the hydrogen coverage decreases within the range of the surface carbons remaining hydrogenated, the values of the negative electron affinity show lowering trend, and can be explained by the increase of the surface C-H dipoles. These results may provide meaningful suggestions for designing the nanoscale diamond-based optoelectronic and electron-emission devices. (C) 2008 Elsevier B.V. All rights reserved.