The compressive behaviors parallel to the grain, i.e. zones of low stiffness (damage zones) and an effective area in partial compression, were studied by experiments using Japanese cedar. The test parameters are loaded cross-sectional area, height, and width of cross-section of specimens. In the experiments, it was observed that the damage zones were allocated near the compression plates in all the specimens. The height and width of the cross-section of the specimens have an influence on the damage zones and, hence, on the effective modulus of elasticity of the full height of the specimens. The compressive strength has a correlation with the density of the timber and the width of the annual rings. The effective area in partial compression can be estimated to be equal to the loaded contact area in compression. In addition, we investigated the deformation of the contact surface during loading. The shape of the surface of the contact area was found to not be flat and the effective contact area between the wood and the loading plate was smaller than the cross-sectional area of the sample. The shape of the surface of the contact area did not change in the elastic region after 7% of the predicted maximum strength.