This paper discusses the impact of single domain realization on the performance of dye removal-photocatalyst of Congo Red. Cerium-substituted cobalt ferrite (CoCe0.1Fe1.9O4) nanoparticles preparing coprecipitation-route with varying annealing temperatures were used in this experiment. The XRD characterization and Rietveld-fullprof analysis showed that the diffraction pattern was in accordance with ICDD no. ICDD 22–1086 showed a face-centered cubic (fcc) structure with a space group of Fd-3 m, which is the pristine crystalline structure of cobalt ferrite. Calculation of the crystallite size D in hkl (311) orientation using the Scherer equation shows that increasing the annealing temperature promotes a larger of the D. The FTIR results show the presence of two typical absorption peaks within the range of v1 ∼ 400/cm and v2 ∼ 600/cm which indicates the absorption of metal oxide belonging to spinel cobalt ferrite. VSM results show that the magnetic properties are modified by annealing temperature. The largest coercive field Hc = 3.62 kOe for the non-annealed (NA) sample indicated the presence of a single domain configuration. TEM results show the presence of fine nanoclusters for the NA sample and disappear after the annealing process, which is indicated to support the presence of a single domain configuration. Micromagnetic simulations were also performed to evaluate the effect of domain configuration (single or multi domain) on the coercive field. Finally, the NA sample presented the highest dye removal performance of 97.47 %, which suggested the contribution of single domain realization in the photocatalytic process. © 2025