In this study, two aspects concerning with the thermodynamics of CaO-SiO2-VOx system were investigated. One aspect is about the effects of oxygen partial pressure on the phase relationships in CaO-SiO2-VOx system at 1873 K, and the other is the measurement of the standard Gibbs energy of formation of the vanadium compounds. In the first aspect, the phase relationships in CaO-SiO2-VOx system at 1873 K under oxygen partial pressure of 1.7 x 10(-9) atm were determined, and the isothermal section diagram was constructed. Furthermore, by comparing this diagram with that under 6.9 x 10(-11) atm, the effects caused by oxygen partial pressure on this system were elucidated. With the increase in oxygen partial pressure, the compound CaV2O4 that exists stably under 6.9 x 10(-11) atm is oxidized to V2O3 and CaVO3. The solubility limit of V2O3 in liquid phase also increases remarkably, which results in the enlargement of the single liquid area. In the second aspect, the standard Gibbs energy of formation of CaV2O4, CaVO3, V2O3, and Ca2Si1-delta V delta O4 (0 delta < 0.1) at 1873 K were measured. The determined values for CaV2O4 and CaVO3 are -1 259 468.9 +/- 5090.5 J/mol and -963 479.8 +/- 3298.6 J/mol, respectively. Then, the limit of oxygen partial pressure for CaV2O4 existing stably at 1873 K is determined to be 4.4 x 10(-10) atm.