Fatigue life of unfilled polychloroprene rubber is characterized in fully relaxing (R=0) and non-relaxing conditions (R=0.2) for more than 15 network structures. Network changes are induced by thermal oxidation ageing of the rubber and characterized especially in terms of crosslink density. For the first time, in the case of unfilled elastomers, we show that an increase in crosslink density leads to a decrease in fatigue life, which cannot be attributed to large changes in strain-induced crystallization processes. The results obtained here are used to determine the relationship between network structure and fatigue properties. In fully relaxing conditions, it is possible to predict fatigue lifetime by considering crosslink density using a theoretical energetic approach. Nevertheless, the same approach does not work in non-relaxing conditions. In the latter case, a new empirical relationship is proposed to link fatigue life to the crosslink density of rubber.