The effect of H 2 O contents, from 2% to 30vol%, in an ammonium fluoride/ethylene electrolyte on the field emission property of anodized TiO 2 nanotubes was investigated. The morphology and composition of the samples were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively, which were relatively independent of the H 2 O content in the electrolyte. The porous TiO 2 films were produced at low H 2 O content of 2–10vol% due to slow chemical dissolution, and were mainly composed of TiO 2 ·nH 2 O complex. Moreover, the well-aligned TiO 2 nanotubes were grown at moderate H 2 O content of 15vol% with a little TiO 2 ·nH 2 O complex. However, with increasing H 2 O content to 20–25vol%, nanotube clusters of TiO 2 with enlarged inter-cluster distance and sharp tops of each nanotube in clusters were formed due to electric field induced dissolution. The optimal turn-on field for electron field emission of TiO 2 nanotubes was obtained as low as 1.9V/μm. It was suggested that the reduced local field screening effect could significantly contribute to the improved field emission property of TiO 2 nanotubes.