Mechanical alloying (MA) starting from elemental powder mixtures was performed on immiscible Al-10, 30 and 50 at % In alloys. Ai and In were finely mixed with increasing MA time and the crystal size of each element became up to 40 nm after MA for 1152 ks. With refinement of the structure, the hardness increased up to 120 H-V in Al-10 at % In alloy, a value larger than that obtained from the rule of mixtures. The melting temperature of In was recognized, by thermal analyses, to fall by about 3 K for mechanically alloyed Al-in alloy, showing the possibility of forming f.c.t In supersaturated solid solution. A new endothermal peak around 420 K, which corresponded with the melting temperature of metastable f.c.c. In, was recognized for mechanically alloyed Al-50 at % In alloy.