Ammonia is a major metabolic product by aquatic organisms. It is highly and acute toxic on aquatic organisms and therefore must be removed from aquatic ecosystem. A new photocatalytic system was designed to remove ammonia in water in this study. Titanium dioxide was suspended by magnetic stirrer and excited by PL type UV-A lamp light to produce free electron and electron hole. The photocatalytic oxidation of ammonia in sea water was measured in the experiment. Ammonia could be oxidized to nitrite and nitrate in sea water. The photocatalytic oxidation rate of ammonia was 0.25 mg/hr in 2 liter seawater with one PL type 9W UV-A lamp of Philips and 2 g titanium dioxide. The photocatalytic oxidation rate of ammonia was increased with salinity when salinity was lower than 12 psu. The photocatalytic oxidation rate of ammonia was stable when salinity was higher than 12 psu. Sodium ion in water could enhance the photocatalytic oxidation rate of ammonia by titanium dioxide. Excited ion of titanium dioxide may be able to be trapped by sodium ion in sea water and therefore enhanced the photocatalytic oxidation rate of ammonia in photocatalytic experiment. However, when salinity is too high, the concentration of sodium ion is over the demand of excited ion or the excited ion trapped by sodium ion may react with electron hole again, the photocatalytic oxidation rate of ammonia did not increase.