Abstract: Through the addition of Ce for inclusion modification, a systematic investigation was conducted to examine the effects of Ce on inclusion characteristics in Nb-microalloyed medium-carbon alloy structural steels. The distribution and morphology of inclusions in both as-cast and hot-rolled specimens were characterized using optical microscopy, scanning electron microscopy, and electron probe microanalysis, while the equilibrium phase transformation pathways of inclusions were simulated through thermodynamic software. With increasing Ce content, progressive modification of Al₂O₃ and MnS inclusions into CeAlO₃,Ce₂O₂S, and Ce₂O₃ was observed, accompanied by a reduction in average inclusion size from 3.9 to 3.0 μm. Concurrently, Ce inhibited the coupled precipitation of primary NbC and MnS through MnS modification, thereby promoting the independent refinement of primary NbC precipitates. When excessive Ce (e.g., 0.029 wt.%) is added to steel, Ce₂O₃ inclusions precipitate extensively. Owing to the substantial capillary forces of the Ce₂O₃ inclusions, aggregation occurs, thereby forming large-sized inclusions. Following hot rolling, the aspect ratio of sulfide inclusions exhibited a significant decrease with increasing Ce content, while oxide inclusions showed minimal variation in aspect ratio. Based on integrated experimental and thermodynamic analyses, for niobium-microalloyed medium-carbon alloy structural steels in industrial production, the optimal Ce modification strategy should prioritize maximum sulfide modi-fication while suppressing Ce₂O₃ formation or minimizing its content to prevent inclusion aggregation.