Ammonium ( NH 4 + ) is an undesirable by-product of photocatalytic nitrate ( NO 3 − ) reduction since it is harmful to aquatic life once it converts into ammonia (NH3). This research investigated the removal efficiency of NO 3 − and for the first time quantified the relationships of initial nitrate concentrations ([ NO 3 − ]0) and photocatalyst dosages on the remaining ammonium ( NH 4 + ) in synthetic wastewater using photocatalytic reduction process with either nanoparticle titanium dioxide (TiO2) or 1.0%Ag-TiO2 under Ultraviolet A (UVA). The experiments were systematically carried out under various combinations of [ NO 3 − ]0 (10, 25, 50, 80, and 100 mg-N/L) and photocatalyst dosages (0.1, 0.5, 1.0, and 2.0 g). The NO 3 − removal efficiency of both photocatalysts was 98.96-99.98%, and the catalytic selectivity products were nitrogen gas (N2), nitrite ( NO 2 − ), and NH 4 + . Of the two photocatalysts under comparable experimental conditions, 1.0%Ag-TiO2 provided better NO 3 − removal efficiency. For both photocatalysts, the remaining NH 4 + was predominantly determined by [ NO 3 − ]0; higher [ NO 3 − ]0 led to higher NH 4 + . Multiple linear regression analysis confirmed the dominant role of [ NO 3 − ]0 in the remaining NH 4 + . The photocatalyst dosage could play an essential role in limiting NH 4 + in the treated wastewater, with large variation in [ NO 3 − ]0 from different sources.