<p>This study aims to investigate the thermal aging behavior and fatigue properties of natural rubber (NR) composites, with hardness levels varied by adjusting the carbon black (CB) content. Mechanical testing revealed that thermal aging increased the 100% modulus but significantly reduced tensile strength and elongation, with the most pronounced deterioration observed in the CB60 sample (60 phr CB). Crosslink density analysis indicated that aging accelerated network formation, especially at higher CB loadings. Dynamic mechanical analysis (DMA) showed that aging decreased damping capacity (tan δ) while increasing hysteresis loss, indicating intensified internal friction and heat generation. Fatigue life testing demonstrated that thermal aging drastically shortened durability, though the extent depended on CB content. The CB40 sample (40 phr CB) retained relatively favorable fatigue resistance, whereas CB60 exhibited the greatest decline. These findings highlight that CB content not only regulates the hardness of NR composites but also directly governs property changes under thermal aging and fatigue. For applications such as bushings, selecting an appropriate CB content is therefore essential to balance strength, damping performance, and fatigue resistance.</p> Graphical Abstract <p></p>