Beech (Fagus sylvatica L.) is currently the most important tree in Slovakia and therefore new ways and methods of its use are being sought. This thesis deals with the influence of thermal modification on the beech wood at various structure levels. To capture the impact of time and temperature of heat treatment we chose the treatment at the temperatures of 160, 180 and 200 ° C for 1,5 and 3 hours. The heat treatment was carried out at normal pressure in an open system without steaming. We observed changes in density, equilibrium moisture content, swelling, anti-swelling efficiency, colour, contact angles of dyiodomethane and distilled water, surface energy and its components and dynamic modulus of elasticity. In order to determine the effect of heat treatment on mechanical properties of clear wood, we found the strength and modulus of elasticity in compression and tension parallel to the grain, bending, torsion and the Poisson ratios μTL and μR. The influence of the heat treatment on construction timber was identified by determination of strength and modulus of elasticity of large-dimensional samples treated at the temperature of 190 °C for 3,5 hours. At the cell wall level we were detecting the DMT modulus of elasticity and adhesion strength on the compound middle lamella and the secondary S2 layer by atomic force microscopy.We found out that wood after heat treatment becomes a new material with an altered spectrum of properties. Its colour changes significantly, the equilibrium humidity drops by 30%, the swelling elasticity decreases significantly, the contact angle of the water increases (wetting decreases), the penetration time of the drop of water to the surface is prolonged. We have also documented changes in strength properties: an increase in compressive strength, a decrease in tensile, bending and torsion strength. The dynamic and static modulus of elasticity of clear wood did not change significantly but they correlated. After the heat treatment, the structural beech timber significantly reduced its strength to the level comparable to spruce timber. Using the modulus of elasticity in bending, heat-treated wood can be non-destructively sorted into higher strength classes. At the cell wall level, we did not detect structural changes after the heat treatment, but we documented the change of DMT modulus of elasticity and we assume increase of DMTM in the compound middle lamella.