A gas sensor based on CuO has been fabricated to detect an ultra-trace H2S gas. The CuO samples were characterized by X-ray photoelectron spectroscopy, X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. The SEM results revealed that CuO exhibited a "grapes" structure made of nanoparticles. Gas-sensing properties of the CuO sensor were also studied. The sensor showed outstanding selectivity and high sensitivity to H2S gas, and achieved a ppb level detection limit. In addition, the temperature and humidity was also found to be significantly influential on the sensitivity of the sensor. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.