The cotton fiber was oxidized to dialdehyde cellulose by sodium periodate oxidation reaction. The oxidized cotton samples having different oxidation level were obtained with adjusting the periodate concentration and oxidation time. The wide angle X-ray diffraction analysis indicated the crystallinity of the cotton cellulose by the slight oxidation increased, whereas the crystallinity decreased significantly with increase in the degree of oxidation of the cotton cellulose. Differential scanning calorimetric curves of the oxidized samples showed that the slightly oxidized cotton cellulose decomposed at a somewhat higher temperature than the original cotton cellulose in the endothermic decomposition stages up to 350 . However, as the periodate concentration and oxidation time were enhanced further, the endothermic decomposition peak of the oxidized cotton cellulose shifted fast to lower temperature. Furthermore, the changes in the structure and crystallinity were also reflected in the mechanical property studies of these oxidized cellulose samples. The mechanical strength of the cotton thread, which was oxidized by periodate at the concentration of 0.0–1.0 mg/ml and oxidation time for 0–3 h, was found to be almost the same as the original cotton thread, but it decreased remarkably when the oxidation conditions became stronger. These results obtained suggest information in understanding the crystalline structure changes of cellulose in periodate oxidation and planning applications of the oxidized cellulose products.