Abstract: The strengthening effect of Al₂O₃ microspheres was investigated in an Al-6061 based metal matrix composite. The strength of the material in the solid solution state was significantly increased by the addition of the reinforcement microspheres. This increase in strength corresponds well to the calculation of a dislocation strengthening model using dislocation densities measured from TEM observations. The strain hardening rate of the material in various aging states is also increased by this addition, which can be accounted for by the combination of the dislocation strengthening model and the model of geometrically necessary dislocations in plastically non-homogeneous material proposed by Ashby.