Magnesium alloys reinforced with ceramic particles at micro-scale sizes have increased hardness and wear resistance. However, to improve their high-temperature strength and creep resistance the particles have to be smaller. The optimum size of particles for Orowan strengthening is a diameter less than 100 nm. With a large surface area compared to their weight, any deagglomeration of the nanoparticles in a magnesium melt is difficult to achieve and so requires additional processing. This study uses power ultrasound to break down the agglomeration and finely disperse the ceria nanoparticles. The materials were cast in a permanent mold, direct chill casting process using ceria nanoparticles with a diameter of 40 nm and ZE10 magnesium alloy. Three materials are compared: pure ZE10, cast with and without power ultrasound, and ZE10 reinforced with 1 vol.-% ceria dispersed with ultrasound. Metallography, hardness tests and density measurements were performed. Compression creep tests at 200°C and constant stresses were also performed and the minimum creep rates determined. Moreover, the stress exponent was calculated and the concept of a threshold stress applied.