The size-dependent cohesive energy of one-dimensional nanowires and single-walled nanotubes of ZnO [E-c(n)] is established based on considerations on surface bond deficit and curvature effects where n denotes atom number per periodic unit consisting of two ZnO layers. It is found that at n similar to 48, both the structures have the same E-c(n) value, which is a critical value for the size-dependent structural stability of the two structures. The model predictions are well in agreement with the first-principle calculation results. (c) 2008 Elsevier B.V. All rights reserved.