Sho-saiko-to extract, a Chinese herbal medicine, is widely used for treatment of chronic hepatitis in Japan. However, it is not clear what conditions Sho-saiko-to extract improves hepatic inflammation and fibrosis. We therefore induced various stages of liver injury in model rats and administered Sho-saiko-to extract. We then evaluated the liver inflammation and liver fibrosis-improving effects of Sho-saiko-to extract. The liver injury model rats were produced by administration of various doses of dimethylnitrosamine (DMN) and Sho-saiko-to extract was administered to these rats. Then the liver inflammation and fibrosis-improving effects of Sho-saiko-to extract were evaluated according to L-asparate aminotransferase (AST), L-alanine aminotransferase (ALT), liver retinoid levels, levels of hydroxyproline, Transforming Growth Factor-beta (TGF-beta), and the liver fibrosis area. These indicators depended on the total doses of DMN. The ability of Sho-saiko-to extract to improve liver inflammation and fibrosis was limited to the following levels of the respective parameters: AST levels (234-264 U/l), ALT levels (208-232 U/l), TGF-beta levels (1102-1265 pg/g liver tissue), hydroxyproline levels (633-719 nmol/g liver tissue), and liver fibrosis area (9.7-10.6 times for normal rat). These findings suggested that Sho-saiko-to extract is effective in the treatment of liver inflammation and fibrosis up to a certain degree of severity, but it produces no improvement in more severe cases.