Abstract: In oxide metallurgy technology, the particle size of externally added TiO₂ critically influences its evolution behavior in molten steel and the subsequent microstructure control effectiveness. In this study, four different particle sizes of TiO₂ (5–10 nm and 25, 100, 280 nm) were selected. Intermediate samplings were conducted at 1, 5, 10, and 15 min after TiO₂ addition. A scanning electron microscope combined with an inclusion automatic analysis system was employed to investigate the evolution of Ti content in steel, as well as the number density, size, and compositional type of Ti-bearing oxides with reaction time. Combined with as-cast microstructure observation, the induced effect of these inclusions on intragranular ferrite nucleation was evaluated. The results indicate that with decreasing TiO₂ particle size, the Ti content in steel increases more significantly, the three-dimensional number density of Ti-bearing oxides becomes larger, the inclusion size becomes finer, and the spatial distribution becomes more uniform. At 15 min after TiO₂ addition, the number density of Ti-bearing oxides in the 5–10 nm group reaches 2.8×10⁶ N/mm³, among which oxides smaller than 1 μm account for 72.3%, significantly higher than those of the 280 nm group (1.6×10⁶ N/mm³ and 38.1%). Ternary phase diagram analysis reveals that smaller TiO₂ particles can accelerate the compositional transformation of Ti-bearing oxides from Mn-Si oxides to Ti-rich oxides, and promote the precipitation of MnS on the surface or at the edges of oxides, forming composite inclusions. In the as-cast microstructure, the area fraction of intragranular ferrite in the 5–10 nm group reaches up to 81.3%, significantly higher than the 22.9% observed in the 280 nm group. These results are attributed to the fact that the reduction in TiO₂ particle size increases the interfacial reaction area and surface energy, which facilitates its dissolution, decomposition, and reprecipitation in molten steel, thereby improving the introduction efficiency and distribution uniformity of Ti-bearing oxides, and consequently enhancing their effectiveness in inducing intragranular ferrite nucleation. This study provides quantitative evidence and theoretical support for the particle size selection and control of externally added particles.