In order to understand the effects of various iron oxides on the properties of melts, viscosity, density as well as sulphide capacities of slag system containing FeOx were summarized and discussed. It is indicated that either thermophysical or thermochemical properties would be modified accordingly with the valence change of iron oxides. Fe2+ and Fe3+ have different functions in slag. Fe2+ existed as a network-breaker, which resulted in the decreasing of viscosity and increasing of sulphide capacities with increasing addition of FeO. However, Fe3+ commonly would occupy the tetrahedron and octahedron positions, and play a network-former role, consequently, cause the viscosity of melts increasing etc. Direct investigations of slag structure or valence state of iron in the melt also support the regular patterns derived indirectly from the various physical-chemical properties. Meanwhile, these patterns of slag system including iron oxides also offer a good reference to the study of other transition melt oxides in slag system.