Abstract: By using the method of torsion pendulum, this paper has examined the Q-1~T curve of Populus deltoides Bartr. cv. "Lux" (ex, 1-69/55) from the rang of 100~300℃, and observed the internal friction peak of glass’s transition of natural lignin. Also this paper has measured the transition temperature (near 0.5 Hz) Ts (or Tg) 215℃. In addition, a method testing the molecular weight of lignin in nature has been presented.