Mineralogical study of LD converter slags was carried out by means of microscopic, EPMA and X-ray examinations and three crystalline phases, namely (i) dicalcium silicate (ii) dicalcium ferrite (iii) spinel phase or ‘FeO’ solid solution were identified. Phosphorus was found to exist only in the solid solution of 2CaO·SiO₂. The rapidly and slowly cooled synthetic slags of CaO-SiO₂-FeO_x system containing 5wt% P₂O₅ were also studied by the same method. The slag compositions with different primary phase area chosen in order to obseve the phosphorus-enriched phase. The results of the CaO-SiO₂-Fe₂O₃ system indicate that silico-carnotite [5CaO·SiO₂·P₂O₅] was formed and it changed to the fluorapatite [Ca₅F (PO₄) ₃] when 5 wt% of CaF₂ was added. On the other hand, in the system CaO-SiO₂-‘FeO’, silico-carnotite was formed only in the Ca₂SiO₄ primary phase area.