Abstract: Recovering the iron (Fe) and phosphorus (P) contained in steelmaking slags not only reduces the environmental burden caused by the accumulated slag, but also is the way to develop a circular economy and achieve sustainable development in the steel industry. We had previously found the possibility of recovering Fe and P resources, i.e., magnetite (Fe₃O₄) and calcium phosphate (Ca₁₀P₆O₂₅), contained in steelmaking slags by adjusting oxygen partial pressure and adding modifier B₂O₃. As a fundamental study for efficiently recovering Fe and P from steelmaking slag, in this study, the crystallization behavior of the CaO–SiO₂–FeO–P₂O₅–B₂O₃ melt has been observed in situ, using a confocal scanning laser microscope (CLSM). The kinetics of nucleation and growth of Fe- and P-rich phases have been calculated using a classical crystallization kinetic theory. During cooling, a Fe₃O₄ phase with faceted morphology was observed as the 1st precipitated phase in the isothermal interval of 1300–1150°C, while Ca₁₀P₆O₂₅, with rod-shaped morphology, was found to be the 2nd phase to precipitate in the interval of 1150–1000°C. The crystallization abilities of Fe₃O₄ and Ca₁₀P₆O₂₅ phases in the CaO–SiO₂–FeO–P₂O₅–B₂O₃ melt were quantified with the index of (T_U − T_I)/T_I (where T_I represents the peak temperature of the nucleation rate and T_U stands for that of growth rate), and the crystallization ability of Fe₃O₄ was found to be larger than that of Ca₁₀P₆O₂₅ phase. The range of crystallization temperature for Fe₃O₄ and Ca₁₀P₆O₂₅ phases was optimized subsequently. The Fe₃O₄ and Ca₁₀P₆O₂₅ phases are the potential sources for ferrous feedstock and phosphate fertilizer, respectively.