In this study, ZnO nanowires (NWs) were synthesized by hydrothermal growth process under the assistant of external electric field. Hydrothermal method is a low temperature and low-cost process for fabrication of single crystalline nanostructures. The ZnO film on Si substrate was deposited first by RF-sputter as a seed layer. Zn(NO3)·6H2O and (CH2)6N4 were then used as reactants in the hydrothermal process. Different direction and strength of electric field were used to investigate the influence of growing of ZnO NWs. It is found that the growth rates and average diameters of ZnO Nws are different while applying the electric field in different directions.