Three different types of chitosan were prepared from red crab shells to study anti-microbial activity of chitosan on pathogenic bacteria, MRSA(Methicillin-resistant. Staphylococcus aureus), Water-insoluble chitosan, whose degree of deacetylation is kept over 90% and molecular weights are 20,000, 500,000, 150,000, 80,000, and 40,000, respectively. Water-soluble chitosan, whose degree of deacetylation is about 48% and molecular weights are 200,000 and 80,000. Water-soluble chitosan, whose degree of deacetylation is 82% and molecular weight is 3,900. The anti-microbial activities of three types of chitosan were investigated by Tube Dilution Technique(TDT) and Agar Plate Smear Method(APSM). And the following conclusions are made ; Chitosan having 5 different types of M.W chitosan (over 90% deacetylation) showed similar anti-microbial activities at over 0.05% concentration. Especially, chitosan having M.W 40,000 150,000 showed the excellent anti-microbial activity. The anti-microbial activity of chitosan was enhanced when the chitosan/acetic add solution was aged for 7days. The anti-microbial activity of chitosan was only shown at chitosan/acetic acid solution. The anti-microbial activity was not detected in chitosan solution dissolved in neutral pH water. Therefore, it can be concluded that the anti-microbial activity was due to NH3+ cationic ion of chitosan in acidic aqueous solution.