Heat treatment is a very common type of wood modification because it improves most of its physical and mechanical properties. However, wood is a combustible material, so it is necessary to know its effect on its fire properties. In the diploma thesis we focused on a summary of theoretical knowledge in the field of hardwood and thermowood. The aim of the work was to determine the effect of thermal treatment of selected deciduous trees on their flammability. The subject of the research was thermally modified deciduous trees: oak, birch, ash and teak. The thesis contains a presentation of the results of authors solving the thermowood flammability problem. The evaluation of flammability was based on a comparison of the fire characteristics of weight loss, heat of combustion and burning rate. The results showed that the heat treatment has a negative effect on the evaluated fire characteristics of deciduous wood - higher values of weight loss, heat of combustion, burning rate and maximum burning rate. The results showed improving fire characteristics when the thermowood is treated at the same time with flame retardants - values such as burning rate and maximum burning rate decreased.