Abstract: The peripheral blood was collected from cobia (Rachycentron canadum) on the 60th day after hatching, and blood cells were studied by light microscope. Five types of blood cells were observed as follow：erythrocyte, lymphocyte, monocyte, neutrophil and thrombus cells. The erythrocyte counts and leukocyte counts of cobia was (2.970.82)109 indmL-1 and (1.390.94)106indmL-1, respectively. The percentages of lymphocytes, neutrophils, monocytes and thrombocytes in total leucocytes, were (53.008.51)%, (17.594.28)%, (4.781.37)% and (24.634.08)%, respectively. Neither eosinophil nor basophil was found. The quantity of erythrocytes was much larger than that of leucocytes. Among the leucocytes, the lymphocytes were the most abundant, while that of monocytes was the least. The immatured and dividing erythrocyte were found. The small lymphocytes with pseudopod and large lymphocytes with the villus were observed also. In addition, the monocyte, neutrophil and thrombu cells were recognized by different shapes of nucleolus.