Abstract: Blood cells play important roles in the immune activities and circulating system of fish, the development of blood cells in liver, spleen, pronephros and kidney and the composition of blood cells in peripheral blood of Trachinotus ovatus were studied. Trachinotus ovatus (25.9) 28.0 cm BL, weight 420.5) 525.2g) were anatomized after one week rearing in lab, the liver, spleen, pronephros and kidney were obtained and gently smeared on glass slides, the glasses were stained with Wright. s-Giemsa dye and the blood cells in different development stages were observed by light microscope.The percentages of primitive erythrocytes in total blood cells counted of liver, spleen, pronephros and kidney were 0.05%, 3.84%, 0.89% and 0.95%, respectively. The percentages of primitive and immature lymphocytes in total blood cells counted of liver, spleen, pronephros and kidney were 0.15%, 2.85%, 2.72% and 3.21%, respectively. The percentages of primitive and immature granulocytes in total blood cells counted of liver, spleen, pronephros and kidney were 0.24%, 2.16%, 4.94% and 3.19%, respectively. The percentages of primitive and immature monocytes in total blood cells counted of liver, spleen, pronephros and kidney were 0.18%, 0.06%, 0.17% and 0.03%, respectively. The q-test results showed that erythrocytes developed mainly in spleen; lymphocytes developed mainly in kidney, pronephros and spleen; granulocytes developed mainly in pronephros; monocytes developed in liver, pronephros; spleen and kidney. The development and morphological changes of thrombocytes could not be distinctly distinguished in the smears of four tissues.The cell constitution in peripheral blood of Trachinotus ovatus were studied by smear and light microscope observation. On the Wright. s-Giemsa dye stained smears of peripheral blood, erythrocyte and four kinds of leucocyte: lymphocyte, thrombocyte,neutrophil and monocyte were recognized. Monocytes were the least leucocytes, less than 1% of total leucocytes. The percentages of lymphocytes, thrombocytes, and neutrophils in total leucocytes were (52.521.65)%, (38.231.86) %, (9.255.64) %, respectively.