Histogenesis of the head kidney was examined in the collected grass carp (Ctenopharyngodon idellus) from 1 to 103 days post hatching (dph). Histological technology was applied to learn the change of the structure of the head kidney and the changes in the distribution of IgM (Immunoglobulin M) positive cells and PAS (Periodic acid Schiff reaction) positive cells in head kidney. The primordium of the head kidney formed at 6dph. And in the primordium several undifferentiated hemopoietic stem cell spread among the renal tubules which derived from the protonephridia. Then, the immune cells increased gradually due to the hemopoietic stem cells’ differentiation from 7 dph to 16dph. Between 17 dph and 103 dph, the renal tubules degenerated gradually and the lymph and haematogenous tissue sustained by the reticulo-endothelial system (RES) became the major component of the head kidney. By 65 dph, the head kidney was histologically similar to mature fishes, formed as a lymphoid- adrenergic tissue. Cells producing IgM as lymphocytes were detected by immunohistochemistry (IHC)stain. A few PAS positive cells were observed by 7dph while several IgM positive cells were occasionally first detected at 17 dph. PAS positive cells and IgM positive cells were both increased with age and finally scattered all over the head kidney. In short, the head kidney in grass carp was almost mature histologically by 65 dph ; as a immune organ, the development of the head-kidney in juvenile grass carp can be divided into three stages characterized by the ⅰ) absence of cells involving in immunity, ⅱ) presence of cells only involved in nonspecific immunity, and ⅲ) emergence and proliferation of cells involving in both nonspecific and specific immunity.