In this study, the effects of ammonia (NH3) were explored for catalytic oxidation of elemental mercury (Hg-0) over CeO2/TiO2, CuO/TiO2, Fe2O3/TiO2, and V2O5-WO3/TiO2 catalysts. In the presence of 50 ppm NH3, the results showed that CuO/TiO2 catalysts could still maintain 98-100 % of Hg-0 conversion in the range of 100-400 degrees C, while the catalytic performance of other catalysts was significantly inhibited by NH3 in the same temperature range. NH3 resistance was observed on CuO/TiO2 catalysts. The catalysts were further characterized by means of XRD, H-2-TPR, XPS, NH3-TPD, BET, and in-situ DRIFTS. For the CuO/TiO2 catalysts, Cu(II) was the primary Cu species that conducive to the strong reducibility properties which beneficial for the catalytic performance. The mechanism of NH3 resistance in Hg-0 oxidation over CuO/TiO2 catalysts was proposed. At low-temperature range (lower than about 250 degrees C), abundant weak acidic sites contributed to the superior Hg-0 adsorption capacity. The Hg-0 oxidation activity was enhanced correspondingly. At high-temperature range (about 250-400 degrees C), the Hg-0 oxidation performance was mainly affected by the oxidation of NH3 and the products. The main product of NH3 oxidation was NO2, which could promote Hg-0 oxidation activity.