The ultrasonic and the combined ultrasonic/mechanochemical treatments were used for the 1% CuO-MgO catalyst preparation by a simple mechanical mixing of the CuO nanopowder with MgO. Results of the XRD and TEM studies show that the ultrasonic treatment significantly increases the CuO nanoparticle size. The CO oxidation study reveals that the impact of the force treatments caused by the ultrasonic and the combined ultrasonic/mechanochemical activation leads to the catalyst deactivation due to the CuO nanoparticles agglomeration.