Objective: To identify and isolate phenolic compounds from Cuspidaria convoluta, and to evaluate their antibacterial activity and synergistic effect with antibiotics. Methods: The crude extract was prepared by maceration with methanol (5％). The dry extract was suspended in water and fractionated successively. The most active extract was selected by its antibacterial activity and its total phenol content was determined by spectrophotometry and by HPLC-MS/MS. Bioactive fractions of the most active extract were separated by column chromatography and evaluated by bioautography. Isolated compounds were identified. Minimum inhibitory concentration (MIC) of these compounds was determined by microdilution broth method, and synergism with antibiotics (ampicillin, gentamicin and oxacillin) was tested by checkerboard and time-kill assays. Results: Coumaric acid, catechin/epicatechin, and luteolin were purified and identified from the extract. There was an increase in the antibacterial activity of antibiotics when they were combined with these compounds. The combination of luteolin and ampicillin had the most potent antibacterial activities. The MICs of oxacillin for each of methicillin-resistant Staphylococcus aureus strains were reduced between 4 and 8-fold when these strains were coincubated with sub-MIC (≤ 1/2 MIC) levels of these compounds, demonstrating that the combination had synergistic effect for all cases. Conclusions: Cuspidaria convoluta contains important pharmacologically active substances that can be used to improve antibiotic efficacy.