Abstract: Non-metallic inclusions in 316L stainless steels manufactured by continuous casting (CC) and additive manufacturing (AM) were quantitatively studied in this work. Electron microscopies with quantitative statistical analysis were performed. It is revealing that both steels contained SiO2-MnO-Cr2O3 inclusions, but those in the AM sample exhibited a much smaller diameter (0.35 µm) and a narrower size distribution than those in the CC sample (0.81 µm). This is due to a high melt pool temperature, and a rapid melting and solidification process during additive manufacturing. In-situ observation experiments results using high temperature confocal laser scanning microscopy (HT-CLSM) showed that inclusions in the CC sample were subjected to collision and agglomeration driven by the attractive capillary forces with a range of 10⁻¹⁷-10⁻¹⁴ N, whereas the inclusions in the AM sample migrated under external fluid flow, i.e., Marangoni flow without coalescence. The suppression of inclusions agglomeration in AM steel was attributed to their sub-micron size. The current findings clarify differences in features (composition, size, distribution, etc.) and agglomeration potency of the same type of SiO2-MnO-Cr2O3 inclusion in the same grade 316L stainless steel manufactured using two routes. The obtained knowledge in this work states that the interfacial phenomenon is one of the key factors to manufacture ultra-clean steels for a sustainable future.