When the moisture content is reduced from the air-dried state, the internal friction of wood increases and the peak in tan δ-frequency curve shifts to the lower frequency. The Young's modulus and tan δ decreases and peak of tan δ shifts to the higher frequency with the increase of temperature at the absolutely dried state. The experimental activation energy was found to be about 1000 cal/mol. It is larger for the broad-leaved tree than for the needle-leef tree. The frequency character of tan δ can be explained by superposing both Eyring's and Newton's viscosity.