Thioflavin (TFT), a benzothiazole fluorescent dye, is able to selectively stain amyloid structures and is mainly used to detect amyloidosis using fluorescence microscopy. In this study, the circulating hemocytes of four species of mollusks, Chlamys farreri (Bivalvia), Mactra chinensis (Bivalvia), Natica janthostomoides (Gastropoda) and Octopus variabilis (Cephalopoda), were observed in order to develop the fluorescence staining method of mollusks haemocytes with TFT. Under fluorescence microscope, haemocytes of four mollusks showed yellow-green fluorescence after treatment with 0.1% TFT. The outlines of the cells were clear; the nuclei and granules in the cytoplasm were clearly distinguishable. The haemocytes of all four mollusks could be divided into two types, granulocytes and agranulocytes, based on whether cytoplasmic granules could be observed. In scallop, clam and octopus, the agranulocytes could be subdivided into hyalinocytes and haemoblasts, while the granulocytes could be subdivided into small granulocytes and large granulocytes according to the size of granules in the cytoplasm. The fluorescence staining method of mollusks haemocytes with TFT is easy to operate, and the samples can be observed for a long time after staining because the fluorescence is not easy to bleach. This study is the first to use TFT in the morphological characterization and classification of mollusks haemocytes, and suggests that TFT fluorescence staining is a good method for the observation and classification of mollusks haemocytes.