A bioassay-guided study of stem bark lipophilic extracts led to the isolation of a bioactive compound from Lasianthus lucidus Blume species related to traditional medicine for treatment of wound infections, bleeding, and fevers. Scopoletin (7-hydroxy-6-methoxycoumarin) could be isolated from stem bark extract yielding 1.4 μmol/g by dry weight. The minimum inhibitory concentrations of extracts and scopoletin proved to be effective against both strains of Pseudomonas aeruginosa ATCC 27853 (AmpC β-lactamase producing strain) and P. aeruginosa DMSC 37166 (clinical strain). They revealed the highest antibacterial effect at 128 μg/ml and morphological changes on bacterial cells were demonstrated by Field Emission Scanning Electron Microscope. The lipophilic extracts and the purified scopoletin from L. lucidus have clear antibacterial activities, especially acting against P. aeruginosa strains. They produce cell lysis, inflated swelling cell walls and cell walls sinking into cells in the same way.