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This study was set to examine the fundamentals of solidification microstructure and room-temperature fracture toughness of different eutectic aluminum (Al) alloys strengthened by T-Al 6 Mg 11 Zn 11 and/or β-Al 3 Mg 2 intermetallic phases. The thermodynamic calculations for an Al-Mg-Zn ternary system assessed three alloy compositions of corresponding to α-Al/T, α-Al/β and α-Al/T/β eutectic compositions, respectively. The designed alloys were prepared by different cast process for changing the cooling rate in solidification. The Al-22.5Mg-23.5Zn and Al-35.5Mg-2Zn alloys exhibited a number of fibrous α phase surrounded by of T-phase and β-phase matrix, respectively. The morphologies of eutectic microstructures were refined by high cooling rate in solidification. The three-phase microstructure of α, T and β phases was often observed in slowly solidified Al-34.5Mg-5Zn alloy, whereas only the α/β two-phase eutectic microstructure was observed in the rapidly solidified alloy sample. The indentation fracture method using Vickers indentation test was applied to measure the room-temperature fracture toughness of the experimental alloys prepared by mold-casting. The α/β twophase eutectic alloys exhibited a low fracture toughness of 1.1 MPa•m 0.5 in comparison with the α/T two-phase eutectic alloy. The lower fracture toughness would be responsible for the brittleness of β-phase matrix in the α/β twophase eutectic alloys.