Carbon monoxide (CO) is known as a silent killer, as breathing in CO gas ambient for long periods with a concentration above the tolerance limit can cause severe health hazards and even death. CO concentration as low as 30 ppm (parts per million) inhaling for few hours can significantly affects the human health. This work focuses on CO gas sensing behaviors of cupric oxide (CuO) nanostructures at relatively lower operating temperature (150°C). The sensors show significant sensitivity for 25 ppm CO gas with fast response and recovery times as well as good reproducibility. Firstly, thin copper films are vacuum deposited on glass substrates at room temperature. Afterwards, these films are thermally oxidized in air, using simple thermal oxidation technique. CO sensing properties of CuO thin film are evaluated for different operating temperatures (150-250°C) and concentrations. The CuO nanostructure shows the maximum CO sensitivity for an operating temperature of 230°C. However, CO response increases with the CO concentration, which is finally saturates for a particular operating temperature. © 2019 IEEE.