Abstract. In this research study of the treatment of ammonia in liquid hospitals waste using activated carbon. This study aims to the effect of activated carbon weight and precipitation time to the treatment of ammonia in liquid hospitals waste. Hospital liquid waste has been taken from Jogja International Hospital (JIH) Yogyakarta, Indonesia. Hospital liquid waste 100 mL is mixed with activated carbon with the varied weight that is 15, 30 and 60 g. After added with activated carbon then stirred with a magnetic stirrer for 15 minutes and a precipitation time of 0.5, 1.0 and 2.0 hours. The next step is the filtrate analyzed ammonia concentrations before and after treatment using UV-Vis Spectrophotometer. The results showed that activated carbon can reduce ammonia concentration in hospital liquid waste. The amount of the active carbon and the time of stirring, the greater the ammonia concentration decreases in hospital liquid waste. The best condition for the decrease of the ammonia concentration was obtained with active carbon and precipitation time is 60 g and 1.0 hours, respectively with ammonia decrease of 95.93%. The conclusion is that activated carbon can reduce ammonia concentration in hospital liquid waste.