Crack resistance performance by oxidation must be considered in rubber product development because cracks are the starting point for fracture. When rubber is stretched extensively, mechanical stresses can break apart its molecular chains, generating radicals and promoting mechanochemical oxidation. This reaction is one of the causes of cracking. However, it is not fully understood how cracks are formed in vulcanized rubber with an inhomogeneous crosslinking structure. Atomic force microscopy (AFM) nanomechanics has been utilized to observe vulcanized butadiene rubber in an elongated state. It was shown that crack generation is related to mechanochemical oxidation, and that the cracks could be clearly visualized as stress-concentrated regions at the nanoscale. Additionally, crack formation is accelerated with increasing elongation of the rubber. This demonstrates that AFM nanomechanics is an effective tool for observation of the generation of cracks associated with mechanochemical oxidation in rubber materials. Display omitted •Crack generation was investigated using atomic force microscopy (AFM) nanomechanics.•Cracks were generated from stress-concentrated regions in the nanoscale.•Elongation and oxygen in the air cause crack generation.•Crack generation was accelerated with an increasing elongation ratio.