The aim of this paper is to investigate the mechanism of hepatocellular damage in rats caused by low serum selenium.Thirty six rats were randomly divided into 2 groups:group A (fed with low-selenium diet from the Keshan Disease area with the content of selenium being 0.017 mg/kg);group B [fed with selenium-supplemented diet and 0.3 mg/L selenium (Na2SeO3) was added to the drinking water].Both were respectively fed for 12 weeks.At the end of the 12th week,the levels of serum selenium,glutathione peroxidase (GPX) and malondialdehyde (MDA) in hepatic tissue were measured;the hepatocellular ultrastructure and apoptosis were observed as well.The levels of serum selenium and GPX in group A were markedly lower than those in group B.MDA level in group A was significantly higher than that in group B.Under the electron microscope (EM),the mitochondria were remarkably changed in group A.The rate of liver cell apoptosis appeared much higher in group A as well.It indicated that the damage caused by selenium deficiency was through the process of oxidation.Selenium deficiency led to apoptosis of hepatocytes where oxidative damage to mitochondria might be the cause.