Thermal modification of wood significantly changes its properties. For the purpose of surface finishing, the main characteristics of wood are its wettability and surface free energy. Spruce wood was thermally modified at temperatures of 70 °C, 105 °C, 120 °C, 140 °C, 180 °C and 210 °C. After the thermal modification process, we determined mass loss of the samples. On the modified specimens, we measured contact angles of distilled water, diiodomethane and formamide by the Wilhelmy plate method with a Kruss tensiometer K100. Using the obtained data we calculated surface free energy of the modified samples by the Owens/Wendt/Rabel/Kaelble method. We found out that with increasing temperature of thermal modification, mass loss increases also. We also stated that the modification temperature influences contact angles of liquids on wood. For instance, we noticed an increase of contact angles of the distilled water with the increase of the modification temperature. Surface energy calculations showed its slight increase also with the raise of modification temperature.