In the present paper, high entropy alloys based on an aluminum-chromium-iron-manganese-nickel (Al-Cr-Fe-Mn-Ni) system were prepared by induction melting and annealed in an inert atmosphere. The resulting samples were analyzed by optical microscopy, scanning electron microscopy, and X-ray diffraction to determine the structural characteristics before and after the heat treatment process. Significant phase transformations and changes in the phase distribution were noticed after the heat treatment process. The results were discussed against the thermodynamic criteria calculations for most promising compositions. Hardness tests were provided for the selected samples to indicate the changes in the mechanical properties between various compositions and between as-cast and annealed samples. Results indicated that the heat treatment process determined a significant hardness increase in one of the studied high entropy alloys.