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We have calculated the ideal strength of aluminum at finite temperatures by implementing an ab initio molecular dynamics (AIMD) method that treats elastic instability, dynamic instability and thermodynamics in a unified first-principles approach.The results reveal significant changes in fundamental mechanical properties of aluminum: (i) the ideal strength drops precipitously with increasing temperature, by as much as 60% at room temperature compared to T=0 K;(ii) the structural instability modes change qualitatively from dynamic phonon softening at low temperature to elastic failure at high temperature;(iii) the highly anisotropic low-temperature tensile strength becomes considerably more isotropic with rising temperature.Finite-temperature phonon calculations predict the disappearance of soft phonon modes near room temperature in excellent agreement with the AIMD results.This work sets key benchmarks for aluminum and opens a new avenue for studying material deformation and strength at finite temperatures.