Arrays of zinc oxide (ZnO) nanorods have been synthesized on silicon substrates by a solution route under an external voltage. They were ～102nm in diameter and grew along the c-axis. Meanwhile, ZnO nanotubes were also obtained. Some comparative experiments showed that applying the external voltage effectively improved the alignment of the ZnO nanorods and enhanced their adherence to the substrates. The role of the external voltage is discussed in terms of the electric field established in the solution and the electrochemical reaction at the cathode. The formation of the nanotubes is attributed to the decrease of the precursor concentration and the disparity between the field strengths at different parts of the ZnO nanorods.