Wood is a natural material, inflammable and combustible material. Considering this fact it is necessary to utilise proper flame retardants. The latest procedures of thermal modification of wood and impregnation with environmentally friendly biocides such as boron compounds were tested. Our purpose was to find out the combustibility of thermally modified wood. Wood was impregnated with 1, 5 and 10 % Topbor based aqueous solutions. The impregnated so as not impregnated wood was then further thermally modified in the vacuum at 150, 170, 190, 210 and 230 °C. For testing, a modified American method ASTM E 69-50 (1965) was utilised. Control samples lost 75 % of wood mass in 4.28 min. At the modified wood, the best results are reported after modification of samples impregnated with 10 % Topbor aqueous solution at 210 °C, where the samples lost only 32 % of the weight in 4.16 min. It was found out that the burning behaviour of the modified wood was reduced only by the addition of boron. When not impregnated, the percentage of loss of wood mass is comparable to the control specimens.