Wood is very vulnerable to shrinkage and swelling. To stabilise it accordingly, we have modified it thermally. We wished to determine the impact of thermal modification on the dimensional stability and resistance to blue stain fungi. Modification of oak and beech panel was performed in a vacuum at a temperature of 170 °C and 200 °C. In 1st case, we modified a previously glued panel. In 2nd case, we modified the wooden elements and glued them together subsequently. Dimensional stability was tested by soaking the samples in water and monitoring the swelling. To assess resistance to blue stain fungi we used a standardised method SIST EN 152. We established that thermal modification increased the dimensional stability of wood already at 170 °C, but even more explicitly at 200 °C. Greater dimensional stability was observed in panels glued together later. The modification increased the resistance of panels against blue stain fungi. In the case of oak wood, it was more effective as it prevented blue staining already at 170 °C, and in the case of beech wood, the resistance to blue stain fungi was indicated at 200 °C.