<p>This study systematically investigates the effects of different La contents (0, 0.011, 0.023 and 0.045 wt pct) on homogeneity of solidification microstructure in sulfur-containing peritectic steel (0.15 to 0.16 wt pct C, 0.021 to 0.022 wt pct S). The results indicate that as La content increases, the typical inclusions in steel first change from Al₂O₃ + MnS to La₂O₂S + MnS and La₂O₂S − LaAlO₃ + MnS inclusions (0.011 wt pct La), and then to La₂S₃ + MnS inclusions (0.023 wt pct La) and La₂S₃ inclusions (0.045 wt pct La). Concurrently, the primary dendrite arm spacing decreases from 75.1 to 58.2, 32.8 and 30.8 <i>μ</i>m; the secondary dendrite arm spacing decreases from 38.6 to 28.3, 15.8 and 14.7 <i>μ</i>m; the average austenite grain size decreases from 233.7 to 122.3, 110.4 and 109.9 <i>μ</i>m; the acicular ferrite content increases from 10.4 to 24.7, 43.2 and 52.6 pct. In addition, the C element segregation ratio decreases from 1.46 to 1.34, 1.26 and 1.22; the S element segregation ratio decreases from 1.82 to 1.67, 1.58 and 1.53; the Mn element segregation ratio decreases from 1.37 to 1.26, 1.19 and 1.16. This is because the effective inclusions of LaAlO₃, La₂O₂S and La₂S₃ promote the δ-Ferrite nucleation, resulting in solidification microstructure refinement. The effective inclusions of La₂O₂S and La₂S₃ promote the α-Ferrite nucleation, resulting in acicular ferrite content rapid increase. Concurrently, effective inclusions' number density increase improves the nucleation efficiency. In addition, δ-Ferrite/Liquid interface number increases with solidification microstructure refinement. As the austenite nucleation core, it limits the single austenite growth space, resulting in austenite grain refinement. The C, S and Mn element segregation ratios decrease as solidification microstructure refinement.</p>