Abstract: Primary extract were extracted from roots of Paeonia delavayi by ethanol and acetone. And then the activity of the extract was screened by 13 kinds of pathogenic bacteria. Inoculating inhibition zone was used to evaluate the antibacterial activity of primary extract and minimum inhibitory concentration (MIC) were determined. Stabilization of ethanol extracts from roots of P.delavayi was detected by treating with different temperature and different pH. The result showed the ethanol extracts and acetone extracts had inhibitory effect to the 5 pathogenic bacteria of 13 pathogenic bacteria, the MIC of 2 extracts to Bacillus cereus, Straphylococcus haemolyticus, Escherichia coli were similar, and MIC to Streptcococcus agalactiae and Vibrio parahaemolyticus had slight difference. The antibacterial activity of ethanol extracts treated with different temperatures and different pH did not change significantly. It means that stability of the ethanol extracts from roots of P.delavayi was good. After macroporous resin treatment, only 100% ethanol eluted fraction showed antibacterial activity.