Bending tests were conducted on oven-dried wood samples (Picea jezoensis Carr.) following treatment with various concentrations of aqueous ethylenediamine (EDA) to investigate the influence of amine treatment on the mechanical properties of wood. Under oven-drying conditions following EDA treatment and a methanol rinse, the densities of wood samples increased at concentrations above 50%, and the Young's modulus decreased at concentrations above 60%. The specific Young's modulus of wood samples decreased at concentrations above 60%, and stress- and strain-at-yield changed slightly at EDA concentrations in the range of 60-70%. X-ray analysis showed that the structures of cellulose changed at concentrations above 60% EDA and confirmed the transformation into cellulose IIII at 70% EDA. These results indicate the possibility that changes in the structure of the cell wall, accompanied by changes in the structures of cellulose microfibrils, contributed to changes in the specific Young's modulus of the treated wood samples. In the same concentration range, changes in the Young's modulus of wood samples increased with increasing relative humidity (RH). This also suggests that changes in the cell wall structure during the treatment contributed to changes in the Young's modulus of wood at different RHs.