Recently, new extraction technology for manganese nodule has been developed as alternative noble metallic resources. It is important to understand thermodynamic behaviors of phosphorus in low basic slag system from the viewpoint of the refining processing optimization. Thermodynamic behaviors of phosphorus in the FeO-MnO-CaO-SiO2-MgO(satd). slag system were investigated at 1723 K with various oxygen potential and slag composition of low basicity. The experimental results for dependence of phosphorus on oxygen potential and slag basicity indicated that the dissolution mechanism of phosphorus into slag of low basicity could be derived as follows; [P]+5/4O2+(O2-)=(PO3.5(2-)) Present experimental results implied that stability of phosphorus in slag would be depended on both of O2- (basicity) and content of Ca2+ in molten slag. The thermodynamic effect of FeO, MnO and Na2O on low basicity on phosphate capacity was discussed.