Abstract: Objective To study the serum ammonia in severe hepatitis and its value.Methods Serum ammonia in 40 patients with severe hepatitis, 20 patients with chronic hepatitis and health adults was measured with DRI-CHEM100 analysator.Results The serum ammonia level of severe hepatitis is much higher than the serum ammonia level of chronic hepatitis (P<0.01) .The serum ammonia level of chronic hepatitis is much higher than that in health adults (P<0.01) .The serum ammonia level of patients with hepatic encephalopathy is a little higher than that without hepatic encephlopaty in severe hepatitis.There's no significant difference between patients with hepatic encephalopathy and with hepatic encephalopathy in severe hepatitis (P>0.05) .There's no significant difference between patients with good prognosis and bad prognosis (P>0.05) .Conclusion Measurement of serum ammonia is helpful in diagnosis of severe hepatitis.