Abstract: Extraction process of total flavonoids from milk thistle (Silybum marianum L.) plants was investigated.With ethanol as extraction solvent, the effect of ethanol concentration, liquid-solid ratio, extraction time and extraction and temperature on the yield of total flavonoids were optimized. The best extraction conditions were determined with single factor test followed by four factor-three level orthogonal experiment. Results showed that the yield of total flavonoids in Pappus was most among five parts of milk thistle. The effect of the investigated factors on the the yield of total flavonoids was in the order as follows:solid-liquid ratio>extraction temperature>extraction time >ethanol concentration. The optimal extraction conditions were that ethanol concentration of60%, liquid-solid ratio of 30∶1 (m L/g) , extraction time of 100 min and extraction temperature of 55℃. The total flavonoids yield was 1.78% under these conditions.