Abstract: A histological examination was conducted to study the ontogenetic development of 3 immune organs of Trachinotus ovatus at early developmental stages. The anlage of head kidney was the first organ to be observed at 2 day-post-hatch (dph), and then followed by spleen (3 dph) and thymus (3 dph). The developmental process for head kidney includes 3 stages which are anlage forming stage, developmental stage of hematopoiesis and degraded stage of urinary function. When the fish became postlarva (18~22 dph), the distribution of renal tubules became less and its urinary function degraded. At 6~8 dph, the hematopoietic stem cells began to differentiate into large amount of immature red cells. At 22 dph, the spleen became larger and more mature, and the inner blood cells were involved in blood circulation. The number and size of thymus cells with weak basophilia grew after anlage of thymus appeared. At 10~13 dph, the basophilia of thymus cells and the number of lymphocyte grew, and the thymus began to differentiate to inner zone and outer zone. During the end of juvenile stage (20~22 dph), the inner zone and outer zone became distinct, indicating the maturity of thymus. The transfer of lymphocyte between thymus and head kidney had not been observed.