To determine the antibacterially effective fraction from Peristrophe japonica (Thunb.) Brem.and to clarify its chemical components. The ethanol extract of Peristrophe japonica (Thunb.) Brem. was separated by macroporous resin column chromatography eluted with water, 80% ethanol and acetone, respectively, to give the fractions A-C. The in vitro antibacterial effects of the fractions were determined by the disc diffusion test. The anti-bacterial potencies of the active fraction was then assessed in vitro by determining the MICs and MBCs. The chemical structures and relative contents of the components present in the antibacterially effective fraction were determined by gas chromatography-mass spectrometry (GC-MS). Each fraction from Peristrophe japonica (Thunb.) Brem showed different levels of antibacterial activity with fraction C (the acetone fraction) being the most active fraction. Twenty-six compounds were identified by GC-MS in the effective fraction, these compounds are fatty acid and its derivatives. The fatty acid and its derivatives is the main antibacterially active ingredients in Peristrophe japonica.