The purpose of this research is to systematically study the influence of artificial aging (T6) on evaluation of microstructure changed and hardness of Al-Al2O3 composite produced by stir casting route. The Al2O3 particle reinforced previously was coated by electroless coating method to improve wetting system between Al-AC8H and Al2O3 when aluminium composites fabricated by stir casting route. The volume fraction of Al2O3 which added into Al melts was various from 2% to 22.5%. The stirring process was carried out at 500-700 rpm to achieve vortex in alumnium melt using graphite impeller for 15 minutes. The prepared aluminium composites then heat treated by T6 treatment with different aging time. Microstructure and hardness after T6 treatment were investigated. It was observed that microstructure changed after T6 while hardness increase after T6 due to the Mg2Si precipitation hardening was present in Al matrix as well as Si eutectic accumulated on the surface of Al2O3. It was found that the optimum hardness occurred after aging for 3 hours.