Nano - crystalline Al-Mg-Mn was synthesized by ball milling technique. Microstructure of these alloys has been studied from X-ray line broadening. The crystallite size of nano - crystalline Al-Mg-Mn system decreases by increasing the Mg content, While the micro-strain, median diameter,, and geometrical standard deviations, increases by increasing the Mg content. Micro-hardness of our system has been investigated by Vickers hardness test. The hardness increases by increasing the Mg content.