Impact of thermal oxidation on mechanical behaviour of polydicyclopentadiene (pDCPD) is studied in this paper. Thermal oxidation is performed over a wide range of ageing temperature, from 20 °C to 120 °C using 60 μm thin films in order to avoid heterogeneous degradation through sample thickness. After several ageing durations, chemical changes were monitored using Fourier-transform infrared spectroscopy (FTIR) and network modification (e.g. glass transition, Tg) was measured using dynamic mechanical analysis (DMA). In addition, tensile tests and fracture tests, based on the essential work of fracture (EWF) concept, were used to study how oxidation affects some mechanical properties of pDCPD. During oxidation polydicyclopentadiene undergoes crosslinking due to the presence of double bonds that leads to a large increase in Tg (from 150 to 225 °C) as well as an increase in rubbery modulus. This increase in Tg results in an increase in maximal stress that can be described using the Kambour relationship. In parallel, an embrittlement of the polymer is observed here with a decrease in both essential and non-essential work of fracture. Finally, it appears that the accelerating effect of ageing temperature can be described using an Arrhenius equation with an activation energy close to 65 kJ/mol for carbonyl formation, maximal stress changes and decrease in fracture energy.