Heat treatment improves dimensional stability and sound absorption properties of wood. However, mechanical properties of wood can be deteriorated during the heat treatment. The effect of heat treatment on the bending strength and hardness of wood for Korean paulownia, Pinus densiflora, Lidiodendron tulipifera and Betula costata were measured. The heat treatment temperature has been investigated at $175^{circ}C$ and $200^{circ}C$, respectively. The results showed that the weight and density of wood decreased after heat treatment. It was found that the density by heat treatment was lower at $200^{circ}C$ than that at $175^{circ}C$. And, MOE increased with the reduced density. On the contrary, MOR and hardness decreased. In all conditions, It was found that there was a high correlation of 1% level between bending modulus of elasticity and modulus of rupture.