We report on a process in which CuO nanopowder was produced in a high yield by adopting ultrasonic in aqueous solution. In our experiment, CuCl2 solution was reacted with NaOH solution and NaNO2, at ambient conditions applying ultrasonic for 5 min. Precipitation was performed by varying the molar ratios of NaOH/CuCl2 and NaNO2/CuCl2. CuO nanoparticles of ~ 5 nm and spherical shape were obtained at the NaOH/CuCl2 of 2.0 and the NaNO2/CuCl2 of 0.097. Without ultrasonication, an amorphous phase was formed at these conditions. This indicates that sonochemical reaction facilitates direct formation of the nanosized CuO particles. In addition, the particle morphology varied from sphere through ellipsoid to needle forms depending on pH. In thick films prepared with the CuO powder for gas sensing, the maximum CO gas sensitivity reached 93 % at the temperature of 250 °C and depended linearly on CO concentration in log scale over the range of 10 ~ 104 ppm.