ZrO2-supported Ru catalysts were prepared by an impregnation method and by NaBH4 reduction from RuCl3 center dot 3H(2)O and a commercial ZrO2 support. The catalyst prepared by NaBH4 reduction showed high activity (around 60%) at 673 K for NH3 decomposition, whereas the catalyst prepared by the impregnation method exhibited low activity (10%) under the same reaction conditions. X-ray fluorescence spectrometry indicated that the catalyst prepared by NaBH4 reduction was free of Cl, whereas the catalyst prepared by the impregnation method contained more than 5 wt % Cl from the Ru precursor. This was main factor that caused the differences in catalytic activity. In addition, the Ru/ZrO2 catalyst prepared by NaBH4 reduction was active for NH3 decomposition without H-2 pretreatment, and its activity was slightly higher than that of the catalyst pretreated with H-2. The transmission electron microscopy and H-2 pulse chemical adsorption results demonstrated that the Ru metal particle size also affected the activity.