Abstract: In this study a case of black-necked swan was observed by histopathology and the disease was diagnosed. The results showed that the typical pericardial effusion, yellow color in liver, bleeding point of spleen were apeared in gross necropsy. Histopathological examination showed hepatic edema, hepatocyte degeneration and necrosis, and a small amount of intracellular basophilic inclusions in hepatocytes; Myocardial fiber rupture, obvious hyperemia, visible monocyte and lymphocyte infiltration. The initial diagnosis was that the black-necked swan was suspected to be infected with a new disease in Tianjin in July 2016: Hydropericardium-hepatitis Syndrome（HHS）, and preventive measures were taken in suspected areas.

Key words: black-necked swan, histopathological examination, hydropericardium, basophilic nuclear inclusion body, HHS