Crystallization behaviors of CaO–SiO₂–CaF₂–La₂O₃ synthesized slag with different basicity have been studied by differential scanning calorimetry (DSC), field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that there were mainly three kinds of crystalline phases (La_7.58(Si_1.048O₄)₆O₂, CaSiO₃ and Ca₄Si₂O₇F₂) precipitated in the slag during the cooling process. Crystalline phase Ca₂SiO₃ formed when the basicity was increased to 1.3. The observations confirmed that the rare earth phase (La_7.58(Si_1.048O₄)₆O₂) precipitated firstly in each slag during the cooling process. With the increase of slag basicity, the crystallization temperature of the rare earth phase decreases while the precipitation peak temperature of CaSiO₃ and Ca₄Si₂O₇F₂ increase. The morphology of rare earth phase is hollow hexagonal which is filled with substrate phase. The optimum condition for crystallization and separation of rare earth phase was obtained. The basicity of the slag should be controlled between 0.9 and 1.1. The morphology of rare earth phases can grow much better after isothermal heat treatment for 4 h at the crystallization temperature of rare earth phases. The mean size of rare earth phase could increase to more than 60 µm by isothermal heat treatment.