Abstract: A dynamic viscoelastic apparatus for "free-free" vibrating beam was developed.It can be used to continually measure the changes of the dynamic Young's modulus and mechanical loss with temperatures for large FRP samples from ambient temperature to 250℃.The method of supporting samples is simple and the judgement on the resonance of samples is easy,Only a few electronic instruments were employed.The repetitiveness of experimental results was good.This-apparatus was easy to be made and cheap.Based on the results obtained from the apparatus, the structure-molecular motion-mechanical property relationships of matrix resins can be established, and the heat resistance of composites can be estimated.These information is particularly useful for designing-application and quality control of composites.