Field-emission properties of ZnO nanoneedle arrays (NNAs) grown on Pt-coated Si (001) substrates by metalorganic chemical vapor deposition at various growth temperatures were investigated. Growth temperature determines the morphology and alignment of NNAs and hence their field-emission properties. The ZnO NNAs grown at 550 degrees C demonstrated better emission properties compared with ones grown at higher temperatures. ZnO NNAs grown at higher temperatures had sharper tips but were relatively less aligned, resulted in inferior field-emission properties. Optimization of the shape and alignment of ZnO nanoneedles is important to obtain better emission properties.