Chitosan oligomers were prepared by oxidative degradation of chitosan (with hydrogen peroxide) under microwave irradiation. This is convenient, friendly and effective method to prepare chitosan oligomers. By 1 H-NMR, IR spectrum, the structure of the products was identified and keeps the same as of chitosan. By LC-MS analysis, it proved that chitosan was decomposed to five fractions, the smallest one about D- glucosamine 3 units (at least).