Abstract: To explore the pathogenesis of nonalcoholic steatohepatitis in model rats. 40 male SD rats were randomly allocated into 4 groups. Normal rats were red with general diet. Model rats were fed with high fat diet. After fed 9 and 14 weeks, the rats were put to death. Blood serum transaminase, glucose, insulin, TNF-α level and liver constitution homogenate MDA, SOD level and liver histology were detected. After 9 weeks, model rats developed insulin resistance, blood serum TNFα level were markedly increased and the livers presented the pathology of hepatocyte steatosis. After 14 weeks, transaminase level, and liver constitution homogenate MDA were increased, SOD level was markedly decreased and livers presented the pathology of inflammation cell infiltration. Insulin resistance and TNFα play a key role in occurrence of hepatocyte steatosis; Lipid peroxidation has intimate relation with inflammation.