The morphological structure evolutions of polyoctenamer upon photo-ageing have been monitored with the aim of correlating the changes of the microstructure with our previous results concerning chemical and molecular evolutions. The influence of the experimental temperature of photo-oxidation has been investigated with regard to the melting temperature of the different crystalline microphases. A critical point was clearly exhibited (10 h of UV exposure at 35 °C). Before this point the degradation occurs according an oxidative process and takes place only in the amorphous phases. During this period, the molecular structure changes to induce rapid cross-linking. After the critical point, the chemical oxidation results from a photolytic process. The extent of the tridimensional network becomes invariant and imperfect crystalline zones are then affected.