Objective:To observe the clinical efficacy of Ruyong Formula on lactational mastitis and its possible mechanism. Methods52 patients with lactational mastitis were treated with Ruyong Formula, with a course of one week. A comparative study was conducted between treatment before and after and between bilateral breasts. The changes on the scores of clinical symptoms (such as breast lump, breast distending pain, obstructed milk, local hot breast skin, aversion to cold with fever) were observed and compared before and after treatment. And the pathogenic bacteria and probiotics in milk of bilateral breasts were qualitatively and quantitatively determined.  Results:①Compared with treatment before, the breast lump was obviously reduced (P<0.05), the breast distending pain was obviously alleviated (P<0.05), the obstructed milk was obviously improved (P<0.05), the local hot breast skin was obviously relieved (P<0.05) and the aversion to cold with fever was obviously improved (P<0.05) after treatment. ②After treatment, the amount of pathogenic bacteria was significantly reduced in the milk of the affected side (P<0.05). ③Before treatment, there were no statistically significant differences on the quantitative Ct values of Straphylococcus, Streptococcus, Bifidobacteria and Lactobacilli in breast milk between the affected side and healthy side (P>0.05). ④For the Straphylococcus-infected patients, on the same side of breast (affected or healthy side), there were statistically significant differences on Ct values of Straphylococcus, Bifidobacteria and Lactobacilli between treatment before and after (P<0.05). For the comparison of D-value on flora Ct values between treatment before and after, the D-value on Ct value of Straphylococcus was increased more obviously on the affected side, which meant that the amount of Straphylococcus was reduced more significantly on the affected side (P<0.05). The D-value on Ct value of Bifidobacteria was decreased more obviously on the affected side, which meant that the amount of Bifidobacteria was increased more significantly on the affected side (P<0.05). For the comparison of D-value on Ct value of Lactobacilli between treatment before and after, there was no statistically significant difference between the affected side and healthy side (P>0.05). ⑤Only one case infected with Streptococcus was found in this study. In the milk of affected side, the amount of Streptococcus showed the decreasing trend and the amount of Bifidobacteria and Lactobacilli showed the increasing trend. ⑥For the non-bacteria mediated patients, on the same side of breast (affected or healthy side), there was statistically significant difference on the Ct value of Bifidobacteria between treatment before and after (P<0.05), and there was no statistically significant difference on the Ct value of Lactobacilli between treatment before and after (P>0.05). For the comparison of D-value on Ct value of Bifidobacteria between treatment before and after, the Ct value of Bifidobacteria was decreased more obviously on the affected side, which meant that the amount of Bifidobacteria was increased more significantly on the affected side (P<0.05). For the comparison of D-value on Ct value of Lactobacilli between treatment before and after, there was no statistically significant difference between the affected side and healthy side (P>0.05).  Conclusion:Ruyong Formula shows satisfactory efficacy in the treatment of lactational mastitis, which can significantly improve the clinical symptoms of patients. It may play the therapeutic role through regulating the mammary gland flora such as reducing pathogenic bacteria and increasing probiotics.