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The stability and mechanical and thermodynamic properties of Al₁₁RE₃ intermetallics (RE = Sc, Y and lanthanide La-Lu) have been investigated by combining first-principles and Debye model calculations. It was found that the formation enthalpies of the Al₁₁RE₃ intermetallics are all negative, indicating that they are stable; moreover, the experimental values of Al₁₁La₃ and Al₁₁Ce₃ are in good agreement with the predicted values, which are −0.40 kJ/mol and −0.38 kJ/mol, respectively. The calculated results of the mechanical properties reveal that the Young’s modulus <i>E</i> and shear modulus <i>G</i> of Al₁₁RE₃ (RE = La, Ce, Pr, Nd and Sm) intermetallics are obviously greater than that of Al, implying that the stiffness, toughness, and tensile strength of them are significantly greater than those of aluminum, and that they, as strengthen phases, can effectively improve the mechanical property of aluminum alloys. The Poisson’s ratio <i>v</i> of Al₁₁Sc₃ (0.37) is the largest, and the heterogeneity is obvious. All the Al₁₁RE₃ intermetallics can enhance the thermostability of the aluminum because of their lower Gibbs free energy <i>F</i> in the range of −5.002~−4.137 eV/atom and thermal expansion coefficient <i>α</i> of Al in the range of 2.34~2.89 × 10⁻⁵/K at 300K, as well as higher entropy and constant volume-specific heat than aluminum at finite temperatures. With an increase in the atomic number, different change trends were observed for the formation enthalpy Δ<i>H_f</i>, bulk modulus <i>B</i>, Young’s modulus <i>E</i>, and shear modulus <i>G</i>. This paper can provide ideas and help for designing a high-performance, heat-resistant aluminum alloy.