The process of thermal modification of wood increases dimensional stability of wood. At the same time, its mechanical characteristics are decreased and its colour changed. The changes are highly dependent on the temperature of modification. The procedure can be controlled by some parameters (temperature and time) to reach the acquired degree of structural changes in wood cell walls. However, these parameters vary as regards to the type of wood. We thermally modified 6 national wood species: Scots pine, Norway spruce, fir, chestnut, hornbeam and beech. Half of them were thermally modified at temperatures of 150 °C, 170 °C, 190 °C and 210 °C. We were studying the influence of modification temperature on mass loss and a colour change. We established that the loss of mass increases with a modification temperature and at the same time, the wood darkens. Darkening of wood during modification very much depends on of wood species. At a temperature of 210 °C mass loss of hornbeam wood reached 16.27 %. We also established, that at some species, the change of colour occurs at lower temperatures (170 °C), whilst the hornbeam darkens for 39.7 % at 210 °C. With colour measurements of sanded surfaces we established that the interior of wood exhibited lower colour changes than the wood's surfaces.