Abstract: 360 juvenile carps were allocated to 3 treatments. Each treatment containing 120 was randomly divided into 4 groups, respectively fed with the purified diets containing selenium 0, 0.15, 0.30 and 0.45mg/kg . The test indicated that the morbidity and mortality of the test groups were negatively correlated to the level of selenium in the diets. The morbidity and mortality were respectively 46.7%, 33.3%, 13.3%, 0% and 26.7% 16.7%, 6.7%, 0%. The diseased fishes exhibited some specially pathological changes, such as "thin back disease" and lordosis. Histologically, the anaplasia of the organs and tissues was obvious, such as nutritional myopathy, nutritional hepatopathy, pancreatic degeneration and necrosis and so on. Ultrastructurally, in addition to the mitochondria of the cardiac muscle cells, the liver cells and renal tubular epithelial cells, were swollen. The disintegration and lysis were noted in the cristaes of the mitochondria which formed vesiculation. The activity of the serum AST and ALT rose, the activity of the serum GSH-Px and the serum SOD decreased, whereas the serum MDA concentration raised.