The antioxidant 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) group was covalently introduced into polydicyclopentadiene (PDCPD) through ring-opening metathesis polymerization (ROMP) copolymerization of dicyclopentadiene (DCPD) with a TEMPO-derived norbornene comonomer. The thermal oxidation of the resulting thin films was monitored by ThermoGravimetric Analyses (TGA) and Fourier-Transform Infra-Red spectroscopy (FT-IR). This new PDCPD stabilized by immobilization of the TEMPO antioxidant shows a better thermo-oxidative stability at 60°C under air than an industrial formulation of PDCPD stabilized with 2,6-di-tert-butyl-4-methylphenol (BHT). Impact of thermal oxidation on mechanical behavior of both formulations have been studied by tensile tests and fracture tests, based on the essential work of fracture (EWF) concept. The TEMPO-functionalized PDCPD offers a promising alternative to BHT-stabilized PDCPD with comparable ductility but slower decay and better cracking resistance, confirming the benefit of the TEMPO antioxidant in PDCPD formulation when chemically bound to the matrix.