Abstract: Pathological changes of tissues and blood of the banded grouper, Epinephelus awoara, infected with the copepod, Caligus nanhaiensis, were reported. The copepod is harmful to the host as it feeds on the tissue of the gills, resulting in the breaking of the gills. The hssue proliferation was not observed in the damaged gills, but the increase of mucus secretion observed. Blold vessels were broken resulting in hemorrhage, and the end of gill filaments became pale. As the blood indices of diseased fish, the quantity of erythrocytes per unit volume became less than the health fish, but the level of serum calcium content was higher than the health. Significant changes in contents of potassium, sodium, urea nitrogen, glucose, cholesterin, albumen,total protein in serum were not observed.