An intimate relation between structure and mechanical properties of regenerated cellulose (Rayon, Benberg, etc.) has been investigated by measuring the humidity and temperature dependencies of infrared spectra, X-ray diffraction, and dynamic viscoelastic properties. At first the dynamic viscoelastic property and the infrared spectra were measured simultaneously during the increasing relative humidity at a constant rate (2% relative humidity/min) at room temperature. The Young's modulus was found to decrease remarkably around 40% relative humidity, where the content of the absorbed water increased largely as evaluated by the infrared spectral data. The water was considered to play a role as a plasticizer. Second, the temperature dependence of dynamic viscoelastic property was measured for the regenerated cellulose in a wide temperature region under the atmospheric environment. When the sample was heated from -150 degreesC, the anomalous phenomena could be observed in the temperature region of ca. -40 degreesC to room temperature: the Young's modulus was diverged, and the intensity and peak position of the X-ray reflections were shifted and the infrared absorbance of the water molecule increased. When the liquid paraffin was pasted on the surface of the cellulose sample, such anomalous phenomena were not observed. The temperature region of these anomalous changes was found to correspond to the region where the absorbed water molecules changed the aggregation state drastically from solid ice to liquid water. In other words, the change in the aggregation state of the absorbed water is considered to affect the mechanical behavior of the regeneated cellulose quite seriously.