Development of the thymus of hatchery-reared barramundi larva was examined using lightmicroscopic histological method. The larva samples were collected continuously from hatching to the day 40th thereafter at 12 hrs. interval. The fish samples were fixed in 10 percent phosphatebuffered formalin solution, serially sectioned and stained with haematoxylin and eosin. The stained sections were observed under light microscope to detect the changes of thymus organ. The time-serial observation showed that thymus germination appeared in fish at the day 10th, and the thymus seems to be morphologically completed at the day 14th after hatching. However, the thymus was only developed completely on cytology feature, and therefore may be functionally matured at the day 30th with the clear differentiation of the cortex and medulla regions with matured T-Iymphocytes. The studied results indicated that vaccination of hatchery produced barramundi may only result in good protection if the application is conducted to fish when fish is older than 30 days of age, when the thymus is structurally and functionally matured.