Crustacean haemocytes play important roles in the host immune response including recognition, phagocytosis, encapsulation melanization, cytotoxicity and cellcell communication. The various types of haemocytes are responsible for a number of defense mechanisms. Thus, the morphology and classification of haemocytes are the basis of crustacean immunology. Generally, classification of the haemocyte types in decapod crustaceans is based mainly on the presence of cytoplasmic granules and relative size of granules. In this paper, Giemsa's staining, hematoxylin and eosin(H.E) staining and electron microscope techniques were used to classify the haemocytes of Procambarus clarkii. And three types of haemocytes, including hyaline cell(HC), semigranular cell(SGC) and granular cell(GC), were observed, accounting for 19.9%，34.1%，46.1% respectively. In addition, the ultrastructures of haemocytes turned out to be most easily recognized under the electron microscope. The hyaline cells were generally the smallest cells with the highest nuclearcytoplasmatic(N/C) ratio, containing few granules, while the semigranulocytes and the granulocytes had increasing amount of granules inside their cells. Afterwards using in vivo and in vitro incubation methods， the haemocyte immunology responses of P. clarkii were investigated, with Sephadex A25 chromatography beads as the antigens. Meanwhile, the cellular recognition mechanism during the immune responses was also preliminarily discussed. When the Sephadex A25 beads were transplanted into the body of P. clarkii for a certain period of time, a large number of haemocytes, which were almost recognized as semigranular cells with slice technique, encapsulated the antigens. It was proved that semigranular cells played a prominent role during the encapsulation process. Therefore, the semigranular cells were the cells active in encapsulation responses. Besides, the results also laid a scientific foundation for the further study of cellular immunological mechanism and antidisease ability of P. clarkii.