The second-order nonlinear optical susceptibilities of ZnO single-walled nanotubes (SWNTs) were investigated by density functional theory. Both the zigzag and chiral ZnO SWNTs exhibit second-order nonlinear optical response while the armchair ZnO SWNTs have no response of second-order nonlinear optical behaviors. The d(33) is the largest component of second-order nonlinear optical susceptibility for both zigzag and chiral ZnO SWNTs, and it decreases as the diameter increase for zigzag ZnO SWNTs. The second-order nonlinear optical susceptibilities mainly originate from electronic transitions from O 2p states in valence bands to Zn 4p states in conduction bands. (C) 2012 Elsevier B. V. All rights reserved.