Change in structure of filler reinforced acrylonitrile-butadiene rubber due to thermal degradation and dynamic fatigue was studied by dynamic viscoelastometry. The glass transition temperature increased with the progress in thermal degradation, but decreased in dynamic fatigue. Furthermore, both the peak value and the full width at half maximum of loss tangent at the glass transition temperature decreased with the progress in the former case, but increased in the latter. The results show effectiveness of dynamic viscoelastometry in degradation tests of filler reinforced rubbers.