Vulcanized natural rubber (NR) under quiescent thermal oxidation aging and high temperature fatigue loading with small strain amplitude was investigated by the infrared spectroscopy, scanning electron microscopy, and positron annihilation lifetime spectroscopy measurements. IR results demonstrated the thermal oxidation degradation process of vulcanized NR at 85 degrees C. During high temperature fatigue loading, nanoscale cracks and voids that are generated by the combined impact of thermal oxidation and cyclic loading were detected. Further investigation suggests that the nucleation effect of dissolved vapor and gas in the low molecular weight domains of the NR under fatigue loading accounts for the appearance of nanocracks. This work provides some new insight into the crack initiation mechanism of NR during high temperature fatigue loading, which has not been clearly understood. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012