The storage modulus and the loss tangent of Sitka spruce wood (Picea Sitchensis) in the longitudinal direction at various moisture contents were measured at 20 degrees C, and the effect of adsorbed water was investigated by using a uniaxial rheological model to eliminate the contribution of matrix swelling. The largest Value for Young＇s modulus of matrix was obtained at around 8% moisture content. The rearrangement of matrix molecules accompanied by the adsorption of hydrated water increased the value of Young＇s modulus up to about 8% moisture content, whereas the plasticization of matrix molecules by the adsorption of dissolved water decreased it at above 8% moisture content. The loss tangent of matrix had two peaks at similar to 1% and 20% moisture contents. It was considered that the former was due to the motion of the adsorbed water itself and the latter to the relaxation related to the micro-Brownian motion of matrix substances, especially hemicelluloses.