Abstract: The interfacial phenomena in mold have a great impact on the smooth operation of continuous casting process and the quality of the casting product. In this paper, the wetting behavior of CaO–Al₂O₃-based mold flux with different BaO and MgO contents was studied. The results showed that the contact angle between molten flux and interstitial free (IF) steel substrate increased from 62.4° to 74.5° with the increase of BaO content from 3wt% to 7wt%, while it decreased from 62.4° to 51.3° with the increase of MgO content from 3wt% to 7wt%. The interfacial tension also increased from 1630.3 to 1740.8 mN/m when the BaO content increased, but it reduced from 1630.3 to 1539.7 mN/m with the addition of MgO. The changes of contact angle and interfacial tension were mainly due to the fact that the bridging oxygen (O⁰) at the interface was broken into non-bridging oxygen (O⁻) and free oxygen (O²⁻) by MgO. However, more O⁻ and O²⁻ connected into O⁰ when BaO was added, since the charge compensation effect of BaO was so stronger that it offset the effect of providing O²⁻.