Abstract: The ultrastructure of hemocytes of Penaeus chinensis, P. japonicus, P.penicillatus and P. monodon, was studied. Four Kinds of blood cells were distinguished according to their morphology, structure and the ultrastructure of granules. The hyaline hemocytes contain characteristic granules, and may be active in phagocytoses. The small-granule hemocytes which contained small rounded electrondense-granule and large striated granules may play a Key role in the immune reaction of the prawns. The large-granule hemocytes which contained large electrondense homogeneous granules may be involved in the defence of the host. The plasma-like cell which did not contain granules is brimmed with laminal rough endoplasmic reticulum.