In this work, ZnO nanorod (NR) arrays were grown by a hydrothermal method on a ZnO seed-layer-precoated Si (100) substrate. The average diameter and length of the obtained ZnO NRs with a high aspect ratio of 33 are 50 nm and 1.65 µm, respectively. Ultraviolet (UV) metalsemiconductor-metal photodetectors (PDs) were fabricated by plating Au interdigitated electrodes with finger spacings of 100 and 200 µm on the ZnO NR arrays. Illumination and electrode areas were half of the same work area for the two PDs. All measured current-to-voltage curves under UV illumination at 380 nm and in the dark show ohmic contact nature at the interface between Au metal and ZnO NR arrays. Here, the PD with 100 µm finger spacing presented a high photo-to-dark current ratio of 258.2 at 1.0 V bias, and its optical responsivity at 380 nm was about two orders of magnitude larger than that at 450 nm with bias ranging from 0.1 to 5.0 V.