The influence of temperature and time of thermally modified fir wood on changes of combustion heat and ash values was observed in the bachelor thesis. The wood was thermally modified at 20, 100, 150, 200, 220, 240, 260 and 280 ° C for 1, 3 and 5 hours. In the untreated and thermally modified samples, heat of combustion and ash were determined. The results show that the heat of combustion increases with the time and temperature of the fir wood's thermal modification. In the thermally untreated wood sample (20 ° C), the combustion heat value was 19,443 J.g -1, the highest combustion heat value of 26,184 J.g -1 was recorded at 280 ° C and 1 hour. The lowest percentage of ash (0.09%) was found in a thermally untreated sample. The highest ash content had a thermally modified sample at 260 ° C and 3 hours.