The Cantor alloy has emerged as one of the most promising alloy systems in recent years, distinguishing itself as a high-entropy alloy (HEA) with exceptional ductility and fracture toughness. This study focuses on a series of CoCrFeMnNi and the addition of 9.43 wt.% of Al in CoCrFeMnNi HEAs, aiming to explore the impact of aluminium (Al) as an alloying element on their structure. These HEAs were fabricated using casting techniques, and their phase transformation was analyzed through X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Hardness values were determined via Vickers hardness tests. The findings reveal that Al significantly influences the alloy’s solidification process and grain growth kinetics. These alterations in microstructure contribute to a notable enhancement in hardness, increasing from 1.798 GPa to 5.007 GPa (178.52% increase). This improvement primarily stems transformation from FCC to BCC crystal structure that leads to strong lattice distortion.