An oriented, crystalline film of cellulose I was transformed into cellulose IIII by supercritical ammonia treatment. These oriented, crystalline cellulose IIII films were converted into cellulose I-beta crystalline film by heating in air for 1 h at 230 degreesC. This transformation process, from cellulose IIII to I-beta, was studied by differential scanning calorimetry and X-ray diffraction methods. During the heating, an endothermic peak was observed at 200 degreesC. This suggests that some hydrogen bonds are broken, which might trigger the transformation from cellulose IIII to I-beta, at this temperature. Analysis of the X-ray diffraction results revealed that cellulose IIII transforms into I-beta in the temperature range 200-217 degreesC. At 217 degreesC, the intrasheet and intersheet distances between cellulose chains in both the cellulose IIII and I-beta crystalline phases are almost identical. Thus, we propose a mechanism account for the cellulose IIII transformation into I-beta by shear of the hydrogen-bonded cellulosic sheets in the be-plane.