We designed three Al-based cast alloys strengthened by T-Al6Mg11Zn11 intermetallic phases using a eutectic reaction (liquid alpha(fcc) + T) in the Al-Mg-Zn ternary system and then fabricated them using solidification processes. Thermodynamic assessment provided target alloy compositions of Al-26.5Zn-21.5 Mg (at%), Al-23.5Zn-22.5 Mg (at%), and Al-13Zn-28Mg (at%) with the alpha-Al (fcc) phase in equilibrium with the T phase with high fractions (>60%). All of the alloys exhibited regularly arrayed fibers of the alpha phase embedded in a T-phase matrix in alpha/T two-phase eutectic microstructures. A slight effect of phase compositions on the fibrous eutectic morphology was observed, whereas the volume fraction of the T phase could be controlled according to the thermodynamic assessments. The fibrous alpha phases had faceted interfaces of {111} planes with a surrounding T-phase matrix, suggesting a low interfacial energy of the alpha/T interfaces. The fabricated eutectic alloys exhibited a high Vickers hardness (>230 HV) and high yield strength at elevated temperatures above 250 degrees C. The Mg-rich alloy with a composition of Al-13Zn-28Mg (at%) exhibited a relatively lower hardness, responsible for the low hardness of the T phase with a Mg-rich composition. The Mg-rich alloy exhibited the plasticity at a lower temperature of 200 degrees C but comparatively lower strength at elevated temperatures above 250 degrees C. These results indicated that chemical composition strongly affected the mechanical properties of the T phase in the eutectic alloys. The present results provide important insights into control of the mechanical properties of Al-based eutectic alloys reinforced by the T intermetallic phase.