There is a special fat cavity in the dorsum of ayu(Plecoglossus altivelis),but no studies have been reported on morphology,structure and composition of the unique fat cavity so far.This study observed the anatomical morphology,microstructure and analyzed the composition of ayu fat cavity.The results showed that the fat cavity of ayu was fusiform and extends from supraoccipital bone to adipose fin,the shape of cross-section exhibits as an inverse-triangle.The weight and length of the fat cavity were(1.95±0.86)g and(9.17±0.82)cm respectively,which account for 4.96% and 53.22% of the total weight[(39.31±5.12)g]and total length[(17.23±0.72)cm]of the fish.In the fat cavity,there were top and down glands,muscles,undifferentiated mesenchymal cell,adipose tissue,and microvasculars.The main components of fat cavity were crude fat(49.10%),water(42.4%),crude protein(6.8%)and ash(1.13%).The percentage of fatty acid was 34.266%,which contains 5 kinds of saturated fatty acid and 6 kinds of unsaturated fatty acid.The highest and lowest fatty acids are palmitic acid(11.800%)and lauric(0.022%)respectively.The total percentage of amino acid was 7.1%,in which the highest content was glutamic acid(1.18%)and the lowest content was methionine(0.10%).This research provided biological basis data for investigating of the fat cavity function,occupy-territoty habit and potential medicinal value.