Abstract: Tissue sections,HE staining and microscopic observation were adopted to investigate the histopathological changes of Trachinotus ovatus with the Cryptocaryon irritans disease. Through histological examination,the results showed that the gills and skin of infected fish were suffered with direct mechanical injury.When the fish under the hypoxia condition,the Cryptocaryon irritans may produce toxins,or cause bacterial or viral infection,thus cause indirect injury of liver,spleen and kidney in different degrees.The injure had following performances:the gill function was seriously blocked,the fatty liver degeneration was severe,hepatic cord was disordered,blood sinus shrunk or disappeared;histological structure of spleen was unclear and had symptoms such as congestion,hemorrhage and necrosis;renal structure was unclear,renal tubule and renal corpuscle were shrunken,renal tubule showed fused cells(adhered),interstitial space was loose and expanded,and local cell died and had fatty granules.