Abstract: The ultrastructure of peripheral blood cells of Alligator sinensis was studied by using transmission electron microscopy. The results showed that the peripheral blood consists of the following cell types: erythrocytes, neutrophil, eosinophil, basophil, monocytes, lymphocyte and thrombocyte. Electron micrographs revealed that the volume of all kinds of blood cells are smaller than those of other reptiles species and all kinds of cells are capable of amoeboid movement and phagocytosis in different extent. The feature of erythrocytes is in the shape of long shuttle and the ratio of its longer diameter to shorter diameter is over 3.0. There are several immature blood cells in the peripheral blood. It has been discovered that the erythrocytes in the peripheral blood had generally developed in certain extent, but some hadn′t finished that last development. The process of the development from immature cell to mature cell of monocytes has been discovered in the peripheral blood, and the changes of the cell′s ultrastuture from infantile type to mature type are described. The various neutrophils are in early infantile type, middle immature type, later immature type and mature type. The ultrastucture and character of neutrophil during every growth period have been described. The relation between azurophilic granule cell and neutrophil has been discussed also.