To optimize CaF 2 content in highly basic CaO-18%Al 2 O 3-SiO 2-10%MgO-CaF 2 (%CaO/%SiO 2 = 6, denoted as C/S = 6) refining slags used for the production of Al-killed duplex stainless steel with high cleanliness demand, the effect of CaF 2 content on the viscosity and refining ability of the slags were studied and compared with typical CaF 2-free highly basic CaO-30%Al 2 O 3-SiO 2-10%MgO (C/S = 6) slag. The effect of CaF 2 addition in decreasing slag viscosity becomes less obvious with increasing temperature and CaF 2 content. When CaF 2 content exceeds 10%, slag viscosity only marginally decreases with further increasing CaF 2 content. Both monoxide-CaO and monoxide-MgO phases are precipitated in all the CaF 2-bearing slags. CaF 2 addition slightly increases monoxide-MgO precipitation, but dramatically decreases monoxide-CaO precipitation. Viscosities of the CaF 2-bearing slags were also theoretically calculated and good agreement with the measured values was observed. Moreover, the 6% CaF 2-bearing slag has very close viscosities above 1 833 K but much lower viscosities below 1 833 K, compared with the CaF 2-free highly basic slag. Further evaluation of the 6% CaF 2-bearing slag on steel cleanliness confirms that 6% CaF 2 addition is sufficient for the highly basic CaO-18%Al 2 O 3-SiO 2-10%MgO-CaF 2 (C/S = 6) slag. The mechanism of CaF 2 in decreasing the viscosity of CaF 2-bearing slags was discussed from the viewpoints that CaF 2 behaves as a network breaker and that CaF 2 suppresses the precipitation of solid phases. The first aspect was identified to play a much greater role in decreasing slag viscosity.