The field emission properties of the randomly oriented and the well-aligned ZnO nanorods synthesized by a thermal evaporation method were investigated in detail. It was found that the field emission of the well-aligned ZnO nanorods is seriously affected by the screening effect. While, compared with the ZnO arrays, the field emission measurements on the randomly oriented ZnO nanorods show a lower turn-on filed and a larger field enhancement factor, which are attributed to the weak screening effect and the surface perturbations. These features make the random ZnO nanorods as a competitive candidate for field emission flat panel display.