An indirect fluorescent antibody technique(IFAT) has been developed to detect Hematodinium sp. in the haemolymph of swimming crab and mud crab. The polyclonal antiserum was derived from rabbits immunized with trophonts of Hematodinium sp. in the diseased Portunus trituberculatus. Antiserum titer, detected by indirect ELISA, was 7680 after reacted with the haemolymph of healthy swimming crab. 18 Samples, including cultured swimming crab, mud crab and wild-caught swimming crab, were tested for Hematodinium sp. by microscopic observation, PCR and IFAT. And the detecting positive rate was 33.3%, 77.8%, 77.8% separatly. The positive compatibility rate of the PCR and IFAT was 100%. Results showed that Hematodinium sp. was speccific labelled with bright fluorescence under UV epifluorescence ,while the haemocytes remained dark. Three different life stages, the trophont , dinospore, sporont of Hematodinium.sp, were also detected by IFAT. In conclusion, IFAT is rapid and specific for the detection of Hematodinium sp., and it’s a practical method for the epidemiology investigation of marine crab and study on the life cycle of Hematodinium sp.