AbstractThis paper investigates the effect of heat treatment temperature on the stiffness and strength properties of Douglas fir (Pseudotsuga menziesii Franco) and common alder (Alnus glutinosa Gaertn.) woods. Two temperatures of heat treatment were used: 165 and 210 °C. The effects of dynamic elasticity modulus, static elasticity modulus, impact toughness, bending strength, and density were evaluated. It is already understood that the mechanical properties, primarily the bending strength, decreases with increasing temperature. In contrast to the favorable stability in shape and dimension that was achieved, the changes in the woods’ properties with temperature were mostly negative. Higher heat treatment temperatures corresponded with lower stiffness and strength properties. For higher temperature treatments, above 200 °C, deterioration of the tested properties was noticable as a result of the significant changes in the wood chemical structure. Even the positive effect of the equilibrium moisture decrease was not able to counterbalance the unfavorable changes. Moreover, it was observed that as the hemicellulose content is higher in alder wood, density, static bending strength, and toughness all decreased steadily at high temperatures, compared to Douglas fir wood.