Nb-Co-Ce/Al2O3 catalysts prepared by impregnation, sol-gel method, and co-precipitation were examined for elemental mercury (Hg-0) oxidation in a simulated coal combustion flue gas. The catalysts were characterized by SEM-EDS, BET, XRD, FTIR, and XPS techniques. The performances of different niobium-loaded catalysts on Hg-0 oxidation efficiency with regard to preparation method, assistant, reaction temperature, and typical individual flue gas were investigated. The results showed that the Hg-0 oxidation efficiency performed at the highest value for niobium-loaded catalysts at 523 K. SO2 was observed to have a negative effect on Hg-0 oxidation. The niobium-loaded catalysts prepared by the sol-gel method and co-precipitation exhibited higher Hg-0 oxidation efficiency than that prepared by impregnation, due to higher specific surface area and more exposure of active sites. However, it also results in these catalysts being affected by SO2 more easily. An enhancing effect of O-2 was observed and an addition of HCl would promote the Hg-0 oxidation further. Results also indicated that synergistic effects between NbOx, CoOx, and CeOx could promote the sustainable capacity of Hg-0 oxidation for Nb-Co-Ce/Al2O3 catalysts. Hg-0 oxidation over Nb-Co-Ce/Al2O3 catalysts is thought to follow a Mars-Maessen mechanism in which lattice oxygen derived from NbOx would react with absorbed Hg-0.