Decomposition of gaseous NH3 from ammonia (NH3)-containing wastewater was explored using Ni-loading Al2O3 catalysts. The thermochemical decomposition of an NH3/steam mixture (wet-NH3) with different steam contents at 873, 923, and 973 K using a fixed-bed reactor under ambient pressure. The present results indicated that the catalysts can be deactivated by the formation of NiAl2O4, which can be thermodynamically generated, and confirmed that the extent of deactivation was greatly affected by the partial pressure of the steam (PH2O). The catalytic activities at 873 K decreased with increasing PH2O, whereas the activity was constant above PH2O of 25 kPa. However, the NH3 conversion was almost independent of the NH3 flow rate and temperature, and similar to 30% of the NH3 was decomposed at each temperature. This study indicated that, even in the presence of steam, this catalyst could decompose NH3 from NH4+-containing water.