Abstract The influence of acrylonitrile (ACN) content and hydrogenation on the fatigue properties of HNBR is investigated. HNBR blends consist of different quantities of acrylonitrile (24, 36, and 44 wt%) and per cent hydrogenation (91%, 96%, and 99%), and a composite of two blends of HNBR with 24 and 44 wt% ACN for an average of 36 wt% . A comprehensive experimental campaign is carried out with fatigue life and crack propagation testing at 120°C. Afterwards, fatigue damage is analysed thanks to both optical and scanning electron microscopy. The results of the three experimental approaches demonstrate that HNBR with median ACN content (36 wt%) and median hydrogenation (96%) has the best fatigue resistance. In general, the fatigue resistance decreases in the following order: for ACN—36 to 44 to 24 wt%, and for hydrogenation—96% to 99% to 91%. The composite blend also has lower fatigue resistance than a regular HNBR blend.