The thesis is concerned with the thermal treatment of beech wood and it is focused on the course of changes in its microscopic structure. Our thesis is subdivided into two main parts, the theoretical part and the experimental part. The opening of the theoretical part deals with beech wood at the macroscopic, microscopic and submicroscopic level. We describe the flaws in it which occur the most, as well as providing a brief overview of its basic physical and mechanical properties. Next the thesis is concerned with thermal wood modification detailing the changes that occur within the chemical and anatomical structure of thermowood, which are subsequently reflected in changes to its overall physical and mechanical properties. The end of the theoretical part is focused on the forms of production of ThermoWood, PlatoWood, RetificatedWood and OHT-Wood and also on the possibilities for using thermally modified wood in practice. In the experimental part, we analysed and evaluated the changes in the microscopic structure of thermally treated beech wood exposed to temperatures raised by 180 and 200°C for a period of 3 and 6 hours by means of a scanning electron microscope Tesan Vega and Tescan Mira. With regards to microscopic changes we concentrated on the occurrence of spherical shapes from condensing compounds of lignin and degradation products of hemicelluloses on the inner surface of the secondary walls of fibres and vessels, on the translocation of condensing compounds in the simple vessel perforations, on the folding of the cell walls, on the production of ruptures in the membranes of the thinned part of the vessel into the radial parenchyma of pith ray, on the warty layer structure, on the formation of fissures, on the dilution of the middle lamella and the change in the compactness of parenchyma cells of pith ray. The mutual colour differentiation spectrum between thermal treated and untreated beech wood was analysed using a spectrophotometer for all four levels of thermal modification.