Various basic data are available for FeOx-SiO2-CaO system in coexistence with metallic iron, which are useful to reductive smelting, but available data are scarce on the effect of minor oxides, especially at higher oxygen potential. In the present paper, the liquidus surface data are reviewed precisely, and recent progress of the experimental and thermodynamic studies is introduced. On the olivine (fayalite) liquidus surface which spreads widely in reductive condition, the addition of CaO or Al2O3 decreases the liquidus temperature, but at higher oxygen pressure more than 10-9 atm, olivine is replaced by spinel (Fe3O4). Above 10-9PO2, the silicate liquid region in the FeOx-SiO2-CaO system is limited by 4 primary surfaces of Fe3O4, SiO2, CaSiO3 and Ca2SiO4, and solid Fe3O4 surface extends substantially by increases in oxygen pressure, Al2O3, and CaO as well as decrease in temperature. However, addition of Al2O3 tends to enlarge the liquid region at SiO2, CaSiO3 and Ca2SiO4 surfaces. The addition of MgO increases the melting temperature substantially regardless the kinds of primary crystal surface. Besides conventional fayalite and calcium ferrite slags, the ferrous calcium silicate (FCS) slag is newly proposed. Instead of triangle FeOx-SiO2-CaO diagram, the phase diagram of CaO plotted against Fe/SiO2 is useful especially for the expression of multi-component system at higher oxygen potential. Also to predict the liquidus surface of these complex systems, the computer aided thermodynamic assessments must be useful method.