To study the pathology of H5N1 avian influenza virus, H5N1 avian influenza virus was inoculated the tail intravenous to the BALB/c male mice and the mice were infected and died. With 1-3 days, the mice began to show acute respiratory distress, decreased activity, less food and water intake, weight and temperature declined. The death of the mice was occured in 2-3days. From 4 to 7 days, the clinical symptom of the mice was resumed. The infected mice showed lung hemorrhage, epicardium and liver necrosis. In histopathology, the main lesion was consolidated pulmonary tissue. The lumina of alveoli and bronchioles were variably filled with protein-rich edema, fluid, fibrin, erythrocytes and cell debris. Extrarespiratory lesions also were found in the mice. There were accumulation of lymphocytes in the epicardium and the liver. Up to the present the epicardium lesion and the hepatic lesion have not been reported in the H5N1 infected mice. The virus titers were not detected in the heart, liver, spleen, kidney and brain tissues by inoculated into embryonated hens' eggs. The virus antigens were not detected by immunohistochemistry in the 6 organ tissues. Our results suggested that the many organ tissues necrosis in the H5N1 virus infected mice might be caused by more cytokine responses, as result multiple-organ dysfunction syndrome, not caused by the virus replication in the organ tissues.