Abstract: With accumulate toxicity test, the effects of 0.5 mg · L^-1 copper sulfate accumulation on ultrastructure of cells in 4 tissues (heart, gill, muscle and hepatopancreas) of Charybdis japonica were studied. The results showed that the copper sulfate accumulation caused more damages to gill and hepatopancreas than to heart and muscle. The injured gill cells had a different ultrastructure, with endoplasmic reticulum dilated and vesiculated, mitochondrial cristaes partially disintegrated, gill fibre cuticles denatured and dropped. The ultrastructural damages to hepatopancreatic cells were characterized by the decrease of microvilli, swelling and breaking up of mitochondria, expansion of endoplasmic reticulum, disruption and vacuolization of nucleus, tumefaction of nuclear membrane and increase of lipid droplets. As for the toxicological changes of myocardial cells, the mitochondria were swelling, having a lot of vacuolus and disintegrating, while the myofibrils were irregular with endoplasmic reticulum dissolved. Although the samples of C.japonica, which had been exposed to 0.5 mg · L^-1 copper sulfate for 16 days, were still alive, ultrastructural damages were found in the cells of gill, hepatopancreas, heart and muscle.