Time-resolved photoluminescence spectroscopy has been used to probe carrier relaxation dynamics of defect emission from ZnO nanostructures of different size and shape. The radiative recombination through the trap states is found to slow down continuously with increasing particle diameters (<= 5 nm) and shows saturation behaviors for larger particles. Intriguingly, the photoluminescence decay for nanorods is observed to be considerably faster as compared to nanoparticles of comparable diameters, which is more apparent for lower energy decays. This effect is assigned to modulation of defect densities and appearance of topological surface states upon shape transitions of spherical nanoparticles to anisotropic nanorods. (C) 2012 Elsevier B.V. All rights reserved.