The classification of haemocytes in Styela clava was studied with flow cytometer. On the basis of cell size and presence of granules, five types of haemocytes were recognized:R₁ is the smallest with a few granules; R₂ is bigger with fewer granules; R₃ is the midium-sized cells with more granules; R₄ is the midium-sized with a few granules; R₅ is the largest cells with more granules. The total amount of haemocytes on the average was about 1×10⁶ cells /ml of haemolymph. The R₁,R₂,R₃,R₄ and R₅ cells constituted 37.15%±1.01%, 15.85%±2.91%, 16.15%±1.58%, 23.65%±3.05%,5.87%±0.31% of the total haemocytes, respectively.Histology study revealed that haemocytes could be divided into haemoblast, hyaline cells, eosinophilic granulocytes, basophilic granulocytes and phagocytes according to cell size and the presence of granules. We also discussed the relations among the cells.