The number of haemocytes per cubic milliliter haemolymph in Eriocheir sinensis was (13357±7196). The percentage of large granular cell (LGC) was 49.8% and its size was (16.6±1.1) μm×(10.2±0.6) μm; The percentage of large and small granular intermediate cell (LSGC) was 20.6% and its size was (13.1±1.2) μm×(8.4±0.5) μm; The percentage of small granular cell (SGC) was 29.3% and its size was (12.4±1.2) μm×(8.1±0.4) μm; The percentage of agranular cell (AGC) was 0.2% and its size was (11.3±1.3) μm×(8.9±1.1) μm. The complete coagulation time of haemolymph in Eriocheir sinensis was (9 7.7±25.3) s, during which only AGC and SGC displayed cytolysis. The main orpho logical and structural changes of AGC and SGC were that they and their nucleisw elled immediately after haemolymph leaving body, the ratio of nucleus to ytopla sm increased rapidly, membranes ruptured and the cytoplasm moved out from the inside of cell. After the complete coagulation of haemolymph, the main changes of AGC and SGC were that their mitochondria and endoplasmic reticulum ruptured, dissolved and disappeared gradually, the substance in the small granules of SGC moved out, the electron density in small granules decreased, the granules vacuolated and disappeared gradually, and nucleus became pyknosis. The cytolysis of LSGC and LGC started 5 min later after haemolymph leaving body and was slower than the cytolysis of AGC and SGC. The main morphological and structural changes of LSGC and LGC were that they and their nuclei swelled, membrane, mitochondria and end oplasmic reticulum ruptured, dissolved and disappeared gradually, large granules in LSGC and LGC vacuolated and disappeared gradually, and nucleus became pyknosis. From the characteristics of AGC and SGC morphological changes after haemolymph leaving body, it was inferred that they released some coagulative factors to get into the coagulative reactions which might be similar to that of vertebrate platelets.