Bicrystalline ZnO nanowires synthesized are composed of two crystals that form a twin structure parallel to the (0 (1) over bar 14) plane with a growth direction closely parallel to [0 (1) over bar1 (1) over bar] and the (2 (1) over bar(1) over bar0) side facet. The twin structure is suggested to be a key factor in leading the axial growth of the nanowire. The photoluminescence spectra of the nanowires show a weak UV emission at 385 nm and a strong green emission at 495 nm. The green emission intensity of the nanowires depends on the different level of oxygen vacancies in the ZnO nanowires. (C) 2003 Elsevier Science B.V. All rights reserved.