First principles calculations of zinc oxide nanoribbons are investigated for zigzag and armchair terminated edges. We present the effects of the atomic relaxation on the shape of the nanoribbons. The electronic properties of the nanoribbons as a function of width and thickness are studied systematically. We found that the stability of armchair edge structures is greater than the zigzag edge configurations. Our results indicate that single layered armchair ribbons are semiconductors, whereas the zigzag counterparts are metallic. However, the electronic behavior changes considerably when the thickness of the ribbon is increased. (C) 2007 Published by Elsevier B.V.