Abstract: A dynamic torsional vibration method was applied to on-line monitor the curing behaviour of hydroxyl-terminated polybutadiene/toluene diisocyanate (HTPB/TDI) system at different temperature. According to the diagram of torque change with time, the curing process was divided into three phases:the first phase was the period from liquid stage to the gel point; the second phase was the period from the gel point to the hardening point; the third phase was the period from the hardening point to the curing end-point. During the third stage the internal chemical cross-linking basically completed while the physical cross-linking point still forming. Therefore, the value of torque was growing slowly till the curing end-point. According to the Flory gelation theory, the calculated △<i<E</i<<sub<a</sub< of HTPB/TDI system is 41.58 kJ/mol, almost the same result calculated with the FT-IR method. FT-TR method can only reflect the first and the second period of the curing process. And the dynamic torsional vibration method can monitor full-range of the curing process. The infrared diffuse reflectance spectroscopy of HTPB/TDI system validates the dynamic torsional vibration method.