This paper addresses the lifetime prediction of polydicyclopentadiene submitted to thermal oxidation, using a multiscale approach. The chemical changes monitored by FTIR revealed the depletion in double bonds and the appearance of carbonyls. Such molecular changes result in the drop of mechanical properties. A kinetic model was implemented to predict the degradation rate depending on external conditions (temperature, oxygen concentration). This model can be completed to take into account the effect of antioxidants (delaying the thermal degradation) and the oxygen diffusion effects (heterogenous degradation) so as to get closer of real cases.