Precipitation-hardened Al alloys exhibit a high hardness and yield stress at 20°C, but these parameters start to drop sharply above 200°C, mainly because of an evolution of the precipitates. The same limitation is encountered with ultrafine-grained Al alloys, due to recrystallization and grain-growth. Al matrix nanocomposites, reinforced by ceramic nanoparticles (Al2O3, SiO2, SiC, TiO2…) distributed both inside ultrafine grains -to hinder dislocation glide- and along the grain boundaries –to prevent their migration and sliding- have the potential to provide a high and stable hardness up to 500 or 550°C. Various techniques were proposed to obtain such materials from powders, each one with its own advantages and limitations. The present study focusses on two of these techniques : consolidation of milled and encapsulated powder mixtures by Equal Channel Angular Pressing (PIT-ECAP) and Spark Plasma Sintering (SPS) after milling. While the thermal stability of ultrafine grained Al alloys issued from ECAPed billets was substantially documented, very few studies considered the thermal stability of Al matrix nanocomposites issued from powders. Howether, the former only exhibits thermally-induced recrystallisation, grain growth and a subsequent drop of hardness, while the latter can, in addition, exhibit thermally-induced cracking. In the present work, Al-Al2O3 nanocomposites were synthetized from ball-milled powders, by PIT-ECAP, performed either at room temperature, with back-pressure, or at high temperature, without back-pressure, as well as by SPS. The initial condition of the consolidated materials (microstructure, residual porosity, presence of cracks, nanoparticles distribution, microhardness and wear properties) were characterized and compared, as well as their thermal stability, not only in terms of grain growth, and resulting drop of hardness, but also in terms of thermally-induced multiple cracking, with crack bridging by Al nanofilaments streched into the superplastic domain, due to a pressurization of entrapped gaz. The amount and initial pressure of entrapped gaz, quite dependent on the chosen process and its parameters, are found to be critical for the thermal stability or the nanocomposites.