CoCrCuFeNi high-entropy alloys (HEAs) have high ductility owing to their unique two-phase microstructure. Phase relations and microstructure evolution were investigated by scanning electron microscopy coupled with energy dispersive spectrometry and differential thermal analysis. These results contributed to clarifying the formation mechanism of the HEA microstructures. The effects of Mg addition as an alloying element in HEAs were also studied, with respect to precipitation hardening, thereby aiming to obtain strength-ductility balanced HEAs.