Water-soluble chitosan and water-insoluble chitosan with molecular weight of 2,000,000, 500,000, 80,000, and 40,000 and more than 90% degree of deacetylation were controlled to evaluate the antibacterial activity of chitosan against a pathogenic bacteria, methicillin resistant Staphylococcus aureus(MRSA), which is being issued in the world. The Shake Flask Method and Modified Shake Flask Method were used to find out the antibacterial activities of 5types of chitosan/acetic acid solution, and the other antibacterial activities test with the cotton filter treated with chitosan /acetic acid solution. Those test methods showed the great differences ,but the results of the antibacterial activites showed the same difference.