Ultrafine-grained (UFG) Al6063 alloy reinforced with 0.8 vol% nanometric alumina particles (25 nm) was prepared by reactive mechanical alloying and direct powder extrusion. Transmission electron microscopy and electron backscatter diffraction analysis showed that the grain structure of the nanocomposite composed of nanosize grains (< 0.1 mu m), ultrafine grains (0.1-1 mu m) and micronsize grains (> 1 mu m) with random orientations. Mechanical properties of the material were examined at room and high temperatures by compression test. It was found that the yield strength of the UFG composite material is mainly controlled by the Orowan mechanism rather than the grain boundaries. The deformation activation energy at temperature ranges of T < 300 A degrees C and 300 A degrees C a parts per thousand currency sign T < 450 A degrees C was determined to be 74 and 264 kJ mol(-1), respectively. This observation indicated a change in the deformation mechanism at around 300 A degrees C. At the higher temperatures, significant deformation softening was observed due to dynamic recrystallization of non-equilibrium grain boundaries. The reinforcement nanoparticles, however, renders the high strength of the material at the elevated temperatures mainly by dislocation pinning.