Dark current in optical sensors has been one of their issues. The performance of these sensors can be improved by reducing their dark current. In this study, we investigated UV sensors based on ZnO nanorods (ZNRs), in particular, their current-voltage characteristics, response and recovery times, on-off current ratio, and responsivity as a function of the device process conditions. The number of seed layer coatings was changed and an oxygen (O₂) plasma treatment was applied to ZNRs. A microwave-assisted growth (MAG) method was used to grow ZNRs. The MAG method is one way to shorten device manufacturing time. The small number of seed layer coating processes played a role in reducing the dark current of the sensor. The O₂ plasma treatment further reduced the dark current. The crystal quality of ZNRs was affected by changes in the sensor fabrication process. Although the O₂ plasma treated device had an increased on-off current ratio, the response time was increased.