The process of thermal modification is used to improve wood properties such as lower hygroscopicity, better dimensional stability and increased resistance to pests. Due to the high prevalence of oak wood in Slovenia, my diploma work focuses on determining the degree of thermal modification of this wood species. Oak wood samples were modified at temperatures of 170 °C, 180 °C, 190 °C, 200 °C, 205 °C and 210 °C in the absence of oxygen. These samples represented standard samples, as we simply determined the mass loss during this process from the difference in mass before and after thermal modification. After modification, the heat-treated samples were analysed by thermogravimetry in the temperature range between 25 °C and 600 ° C in a nitrogen flow. Calibration curves were prepared from the obtained data. They represent the mass loss during TG measurement as a function of the mass loss during thermal modification. Almost all calibration curves showed a linear dependence. The highest correlation coefficient was obtained from the calibration curve in the range between 130 °C and 280 °C, followed by the calibration curve in the range between 130 °C and 290 °C, and the calibration curve in the range between 130 °C and 300 °C. The lowest correlation coefficient was obtained in the range between 25 °C and 130 °C; this dependence was also non-linear. The prepared calibration curves can be used to determine the mass loss during thermal modification of unknown samples.