The kind and quantity of hemocytes in crustaceans are very important for immune defense. There is prominent difference of immune mechanism in the different species and types of hemocyte of crustaceans. It is very important to study the form structure and function of hemocyte for practice. Histology structure , morphology characters and quantity of hemocyte of Panulirus homarus are studied by light microscope (LM) and scanning electron microscope (SEM) . The hemocytes of Panulirus homarus are classified on the basis of whether there are granulesinthecytoplasm, howmanygranulesinit,andthesizeandtheformsontheoutsideofthecells. The results show that there are three types of blood cells on the stained smears : agranular hemocyte , small granular hemocyte and large granular hemocyte. The size of them are 8. 65μm ?8. 08μm, 12. 80μm ?11. 30μm and 15. 90 μm ?13. 50μm respectively. The nucleus2cytoplasm ratio is 4. 88 , 2. 03 and 0. 89 respectively. There are many shapes of hemocyte such as sphere , oval and ellipse. According to the morphology characters , a granular hemocyte , small granular hemocyte and large granular hemocyte are not the blood cells during the different growth periods in the sequence development process of hemocyte , but separate development. The counts of different hemocytes are performed by light microscope. The percentage of agranular hemocyte , small hemocyte and large hemocyte is 57. 1 % ,18. 0 % ,24. 9 % respectively. There are different percentages of the three types of hemocytes in circulation hemocyte of various crustaceans respectively. The differences relate close with the various species , the different physiological capabilities , the different growth and development periods of crustaceans. The pigmentation of three types of hemocytes shows analogy with human white cells by Wright and Geimsa. By the scanning electron microscope observation , all types of hemocytes are not smooth on the surface , where there are a lot of protuberances. It is analogous with the hemocyte and granular hemocyte of humanity. This suggests that the cell structure changes actively and implies their functions with phagocytosis and encapsulation. The granular hemocyte of Penaeus chinensis and other crustaceans have the functions of immune defense. Thus , agranular hemocyte , small granular hemocyte and large granular hemocyte of Panulirus homarus are similar to humanity’s white cells and the other crustaceans in immune mechanism.