Effects of NH4+/NO3- ratios on pH stability of the nubient solution and growth of water spinach in noncirculating hydroponic system were studied under both pH control and non- pH control conditions. With pH control condition, pH of the nubient solutions was adjusted to 5.5 every 3 days throughout plant growth. Two week-old seedlings of water spinaches were grown in soft bucket containing 20L of nubient solution with different NH4+/ NO3- ratios (100:00,75:25, 50:50, 25:75 and 00:100) as treatments. The results showed that water spinach supplied with NO3- as its sole N source (00:100 treatment) grew best under pH control condition, but grew poorly in non-pH control condition. Nubient solution pH of this treatment in non- pH control condition dramatically increased and this possibly affected on the availability of the nubient elements in this treatment, and hence causing poor plant growth. When 100 percent N was supplied in the form of NH4+ water spinach grew worst regardless of pH-control or non-pH control condition, indicating that high NH4+ level was possibly toxic to the plants. In non- pH control condition, the nubient solution pH of 100 percent NH4+ treatment was strongly decreased to less than 4.0. In the NH4+/NO3- ratio of 25:75, the plants grew best both in pH-control and non-pH control conditions. The nubient solution pH of this treatment is stable at pH range of 5.0-5.5, properly for plants growth in hydroponic culture. When propotion of NH4+ further increased, the growth of water spinach decreased. This study indicated that water spinach can grow very well in non-circulating hydroponic system without controlling nubient solution pH throughout the plant growth when 25 percent of NO3- was replaced by NH4+. This replacement also helped to decrease NO3- content in the plants.