The field-emission electrical properties of hydrothermally synthesized zinc-oxide nanowires grown on SiO(2)-Si substrate are reported. Vertically aligned single-crystalline emitters with distinct length L and diameter D are realized by controlling the assisted growth of porous anodic aluminum oxide (AAO) templates. Field-emission measurement revealed that these field emitters exhibited controllable turn-on field E(to) and the field enhancement factor beta. Sample with feature size of L = 500 nm and D = 80 nm was prepared and then used as a basis for examining the size effect. Lower E(to) and higher beta were observed consistently for increasing the aspect ratio LID. The enhanced properties of E(to) = 1.48 V/mu m and beta up to 6100 are achieved for L/D = 53 (L = 1600 nm and D = 30 nm). Optimal characterizing parameters of E(to) and beta will be reached while pursuing extreme L/D practically. Factors such as the geometric limit of AAO template manufacture and the filling efficiency of ZnO into AAO pores will exert influence on the size-dependent effect. (C) 2010 Elsevier B.V. All rights reserved.