Based on the phase equilibria of the CaO-SiO2-VOx system determined experimentally at 1873 K and the oxygen partial pressure of 6.9x10(-11) atm, the isothermal section diagram of the system was constructed. Aside from the simple oxides SiO2 and V2O3, three complex crystal phases CaV2O4, Ca2SiO4 dissolving V2O3, and CaVO3 dissolving SiO2 were found in the present investigated composition range. The solubility of V2O3 in Ca2SiO4 phase can reach 5 mass %, and the CaVO3 phase dissolves about 5% mass SiO2. Furthermore, the thermodynamic activities of VOx and SiO2 in the determined single liquid region were measured by equilibrating the melts with liquid copper. The activity coefficient of SiO2 decreases linearly with the increase in the basicity of the melts and is almost irrelevant to the total content of vanadium oxides. The activities of vanadium oxides increase slightly with the increase in the basicity but are mainly determined by the total content of vanadium oxides in the melts. With the increase in the basicity, the activity coefficient of VO1.5 increases almost linearly, whereas those of VO2 and VO2.5 decrease gradually.