Ammonia is a major metabolic product by aquatic organisms, which is highly toxic, therefore it must be removed from the aquatic ecosystem. A new photocatalytic system was designed to remove ammonia from 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, which 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 increased with salinity when salinity was lower than 12 psu, and the rate was stable when salinity was higher than 12 psu. Sodium ion in sea water could enhance the photocatalytic oxidation rate of ammonia from titanium dioxide. It was because the active ion of titanium dioxide may be trapped by sodium ion 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 active ion or the active ion trapped by sodium ion which reacted with electron hole again. Therefore, the photocatalytic oxidation rate of ammonia did not increase under this high concentration of sodium.