The sinergiliosis of grass carp is a common gill disease caused by parasiting ofSinergasilus major (Mark 1940) which mainly make harms to fish than two years ofmore. The histopathological changes of the damaged gill cells and blood were observedand examined. In the damaged gill, the epithelial, mucous and maesenchymal cellsmass-multiplied and the eosinophils mass-infiltrated, lamellae fused and capillariesatrophied. The ends of gill filaments appear pale and swoller. The other parts of gillfilaments and the lamellae are deformed.As to the blood indices of diseased fish, the quantity of erythrocytes per unitvolume, specific volume of erythrocytes, hemoglobin content, plasma glucose contentand total plasma protein content were all lower than of the healthy fish, but thesubsidence rate of erythrocytes and the quantity of leukocytes per unit volume werehigher than that of the healthy ones. The percentage of lymphocytes and eosinophilsreduce in leukocytes, while the monocytes and neutrophils raised.