Three Co-based dual-phase high-entropy alloys (HEAs) were produced by different manufacturing conditions: arc-melting with Ar protection (Ar-HEA), vacuum induction melting in Al2O3 crucible (Cr-HEA) and vacuum induction melting with 0.5 at. pct Al (Al-HEA), which resulted in different levels of impurity elements and inclusion characteristics. The inclusions that precipitated in different HEA samples were investigated through an electrolytic extraction process and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) characterization. The results showed that Mn(S,Se) inclusions were presented in all three alloys. MnCr2O4 inclusions were presented only in Ar-HEA, pure Al2O3 inclusions were presented in Cr-HEA and Al-HEA, and Mn–Cr–Al–O inclusions were also found in Al-HEA. Thermodynamic calculation software FactSage and Thermo-calc were used to predict the inclusion formations of the HEAs, which showed a good agreement with the experimental findings. The stable inclusions can transform from MnCr2O4 to Mn(Cr,Al)2O4 and then to pure Al2O3 with the increase of Al content. The inclusions in Al-containing HEA are spinel or Al2O3 depending on the content levels of Al and O. It is proposed that the formation of spinel and Al2O3 inclusions can be avoided in liquid HEA when the O content is controlled to be very low, which can result in smaller-sized inclusions. Moreover, the calculated coagulation coefficient of spinel inclusions is close but lower than that of Al2O3 inclusions. The collision growth of inclusions was affected by a combination of physical parameters of HEA and inclusions as well as the inclusion size and amount. Graphical Abstract: [Figure not available: see fulltext.]