Aluminum containing 4 wt.% magnesium was oxidized at a temperature for different oxidation times and analyzed by highresolution electron microscopy. A thin oxidized layer of about 5 μm, which is composed of MgO, forms at short oxidation time and gradually increases. High-resolution microstructures reveal that the oxidized layers are porous regardless of oxidation time. After extended oxidation time, discrete MgAl 2 O 4 particles formed as a result of the reaction of initially formed MgO, liquid aluminum, and oxygen introduced from air through the porous MgO. Furthermore, it is clear by high-resolution lattice images that MgAl 2 O 4 particles are covered with thin Al 2 O 3 , whereas MgO is bonded intimately to aluminum. Therefore, MgAl 2 O 4 particles that form naturally during oxidation are difficult to act as a direct substrate for nucleation of aluminum grains because of the coverage of Al 2 O 3 . In contrast, MgO shows the possibility of acting as a substrate for the aluminum nucleation. The formation mechanism of MgO and MgAl 2 O 4 and their possibility of acting as substrates for nucleation of aluminum grains suggest that atomic level bonding and mismatches of nucleant/nucleus metal should be considered for correct evaluation of the possibility of heterogeneous nucleation of metallic matrix on a potent nucleant.