Field emissions (FEs) of a carbon atom, silicon clusters, graphitic ribbons and diamond surfaces have been investigated within the time-dependent density-functional theory (TD-DFT). The electronic states origin of field emission properties reflecting the atomic geometries and hydrogen (H) termination of edges are elucidated through the analyses of energy band structures, energy distribution of emitted electrons and upsilon- and pi-bonding natures. Findings in this study provide the theoretical basis for understanding the FE mechanism of covalent-bond materials and designing Si- and C-based nanostructured field emitters. (C) 2004 Elsevier B.V. All rights reserved.