We fabricated carbon nanotube (CNT) based chemical sensors for marine applications by photolithography process, where the electrodes were insulated by photoresist exposing only the carbon nanotube sensing section (2 microm gap width) for detection of ammonium ions (NH4+) in 35 per thousand NaCI solution used as artificial seawater environment. The I-V curve of the CNT sensor was measured by sweeping the source-drain voltage from -3 to 3 V and the on/off ratio of the CNT sensor was measured to be 20 when the gate voltage was swept from -5 to 5 V and from these results the CNTs were found to appear as a p-type semiconductor. All of the cocktail solutions prepared for experiment were measured to have -pH 6 which implied 99.9% of NH4+ remained ionized. We successfully detected 10, 100, 1000 nM (0.18, 1.8, 18 ppb) concentration of NH4+ in 35 per thousand NaCI solutions by using the CNT sensor.