Abstract: In order to investigate the haemocyte types and the effect pearl-nucleus-inserting operation in visceral mass on haemocytes in Hyriopsis cumingii Lea, haemocyte morphology were studied by phase contrast microscope, optical microscope, transmission electron microscope and flow cytometry FCM. The present studies showed that two cell populations were found in haemocytes: large cell with high granularity and small cell with low granularity by flow cytometry FCM on light scanttering pattern. Two distinct cell types were identified based on phase contrast microscope: one type of cell was dark and dioptric aberration, while the other was bright and dioptric strong. By Giemsa and H.E staining, cytoplasmic staining were heterogeneous and internal particles were obvious in one type of cell, while cytoplastic staining were homogeneous and internal particles were inexistent in the other type of cell. By transmission electron microscope, we found that the mitochondria, Golgi apparatus organelles were rich and internal particles were obvious in one type of cells, and contrary to the another cells. The results showed that the surface was unsmooth and pseudopodium was obvious in one type of haemocyte, while smooth and unobvious in another by negative staining. All of these indices indicated that the haemocytes were divided into two types: granulocyte and hyalinocyte in Hyriopsis cumingii. When stimulated by pearl-nucleus-inserting operation in visceral mass, the proportion and morphology of haemocytes underwent manifest changes: more haemocyte morphologies, more protuberances, more vesicle-like material, significant increase for density of haemocytes and quantity of granulocytes (PP<0.01) post-operation. The present research indicated that there were obvious changes in the morphology, structures and proportion of populations of haemocyte by pearl-nucleus-inserting operation in visceral mass, which implied immune defense was enhanced after the operation, and the granulocyte played an important role in immune system.