Abstract: Modified zirconia (ZrO₂) were synthesized by high temperature calcination of ZrO(OH)₂ gels digested in alkalic solution; with the modified ZrO₂ as support, ruthenium catalysts for ammonia synthesis were prepared with K₂RuO₄ solution. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction of hydrogen (H₂-TPR), temperature programmed desorption of CO₂ (CO₂-TPD), nitrogen sorption, X-ray fluorescence spectroscopy (XRF), and CO chemisorption; the relationship between the catalyst structure and its catalytic activity was especially discussed. The results revealed that the digestion either in KOH or NH₄OH solution can enhance the surface area of the ZrO₂ support, while digestion in KOH is able to obtain the support with high basicity, which benefits to getting the ruthenium catalyst of high activity. The strong basicity of the modified ZrO₂ support, rather than the high ruthenium dispersion, plays a more important role in enhancing the activity of the supported ruthenium catalyst. For ammonia synthesis under 425 ℃, 5 MPa and a space velocity of 10 000 h^-1, NH₃ outlet concentration over Ru/ZrO₂-KOH is 5.96%, which is 11%, 143% and 103% higher than those of K-Ru/ZrO₂-NH₄OH, K-Ru/ZrO₂-CP and Ru/ZrO₂-NH₄OH, respectively.