The field-emission properties of individual zinc oxide (ZnO) nanowires, grown by a solid-vapour phase thermal evaporation process, were studied in situ by transmission electron microscopy (TEM) using a home-made piezo-manipulator. The results indicate that ZnO nanowires present an outstanding field-emission property with low turn-on voltage and high emission current; the proper linearity of 1/V-ln(I/V(2)) curves basically accords with the Fowler-Nordheim model, and the dependence of the field-enhancement factor β on the distance d between the nanowire tip and its counter anode fits a linear relationship. The investigations show that ZnO nanowires show promise for potential applications as field emitters.