The effect of the nominal Mg content and the milling time on the microstructure and the hardness of mechanically alloyed Al (Mg) solid solutions is studied. The crystallite size distribution and the dislocation structure are determined by X-ray diffraction peak profile analysis and the hardness is obtained from depth sensing indentation test. Magnesium gradually goes into solid solution during ball milling and after about 3 h almost complete solid solution state is attained up to the nominal Mg content of the alloys. With increasing milling time the dislocation density, the hardness and the Mg content in solid solution are increasing, whereas the crystallite size is decreasing. A similar tendency of these parameters is observed at a particular duration of ball milling with increasing of the nominal Mg content. At the same time for long milling period the dislocation density slightly decreases together with a slight reduction of the hardness.