Abstract: OBJECTIVE To study the anti-MRSA (anti-methicillin-resistant Staphylococcus aureus) activity of carnosic acid isolated from Rosmarinus officinalis L.. METHODS Carnosic acid was purified from rosemary extract (contained 72.4% carnosic acid) by silica gel chromatography. Against MRSA ATCC 33592 and eight clinic isolates of MRSA 01 to 08, the minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of carnosic acid were tested by broth microdilution method, and the anti-MRSA effect of carnosic acid combined with oxacillin sodium or ampicillin sodium was designed with checkerboard method and determined by broth microdilution method. RESULTS The content of carnosic acid purified was 98.6%. Against MRSA ATCC 33592 and eight clinnic isolates of MRSA 01 to 08, the MICs range and MBCs were respectively 8-16 μg·mL^-1 and 32 μg·mL^-1, and the fractional inhibitory concentration indices (FICIs) of carnosic acid combined with oxacillin sodium or ampicillin sodium were 0.5 or 0.5-1.0. CONCLUSION Carnosic acid shows obvious anti-MRSA actvitiy, and the anti-MRSA effect of carnosic acid combined with oxacillin sodium or ampicillin sodium is synergistic or additive.