Supercritical carbon dioxide (scCO2) was used to pretreat corncob, cornstalk and rice straw at the temperature of 80-160 °C and pressure of 5-20 MPa, with the raw materials moisture content of 0-75% for the durations of 15-60 min. It was found that the reducing sugar yield of all the three materials showed a maximum with the increase of the moisture content and pretreatment temperature. The optimum experiment result was obtained at 100 °C, 15 MPa with the duration of 30 min and moisture contents of 50%. The reducing sugar yield was 39.6%, 27.4% and 36.6% for corncob, cornstalk and rice straw, respectively, while for untreated materials it was 26.2%, 22.5% and 30.9%. High Performance Liquid Chromatography (HPLC) analysis for hydrolysates of corncob and rice straw indicated that scCO2 pretreatment had a positive effect on the hydrolysis of both cellulose and hemicelluloses. Among the three materials, the reducing sugar yield of corncob was improved the most after scCO2 pretreatment, which could be demonstrated by the X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses.