This paper deals with the regularity of diurnal fluctuation and vertical distribution of non-ionized ammonia concentration at different water depth in the high-yield fish ponds. The concentration of non-ionized ammonia is higher at sunny day than at cloudy day. Early in the morning of sunny day, the concentration is at minimum and it shows little difference in different water layers. In the afternoon, the photosynthesis approaches to the peak. The pH value rises and the concentration of non-ionized ammonia reaches to maximum, though the concentration of the total ammonium decreases. The concentration of nonionized ammonia is the highest in the upper water layer and the lowest in the bottem layer. At dusk when the photosynthesis of phytoplankton becomes lower, the pH value and concentration of non-ionized ammonia decreases in the water layers, especially in the upper and middle layers, though the concentration of the total ammonium increases. At cloudy day, however, the diurnal fluctuation and the vertical distribution of non-ionized ammonia concentration at different water depth have little difference. At windy day, the variation of non-ionized ammonia concentration is also intense, but at fine day with light breeze the concentration among the different water layers has less difference. The effects of photosynthesis of phytoplankton, the respiration and excretion of fish and the decomposition of fish feces on non-ionized ammonia concentration are also discussed.