The temporal evolution of microhardness and Al₃(Er,Zr/Hf) precipitates are investigated in Al-Er-Zr/Hf alloys during annealing at 450 °C and 500 °C. The microhardness of the alloys decreases continuously with the prolonged annealing time due to the coarsening of the precipitates. Different weakening amplitudes are observed because of the disparity of the precipitate coarsening rate that is related to the disparity in their intrinsic diffusivities of Er, Zr, and Hf solute atoms in an Al matrix. The addition of Hf element is beneficial to enhancing the coarsening resistance, thus improving the thermal stability of the alloys. Introducing such elements to improve the thermal stability of precipitates can provide a new idea or choice for the development of heat-resistant aluminum alloys.