In order to study microstructure and mechanical behavior of inhibitors in solid rocket motor at different pyrolysis temperature, Thermogravimetric analysis was conducted at EPDM to obtain its range of pyrolysis temperature, followed by mechanical tests and scanning electron microscopic analysis conducted at EPDM specimens taken at different pyrolysis temperature, thus the reason of the material mechanical behavior transformation was found from a view at microcosmic level. The results show that only slight dehydration and gas fugitive are observed when pyrolysis progress initiates, in the meantime, the material still possess the viscoelastic mechanical properties. While the pyrolysis being exacerbated, segments of the material matrix are dissociated, thus the mechanical behavior of EPDM turns to be brittleness.