Abstract: The rubber expansion joint which was equipped in a nuclear power plant, cracked in the term of service. The macroscopic and microcosmic cracking character, rubber type and mechanical properties were analyzed by means of morphology examination, composition analysis and mechanical performance test. The results show that the delamination of fiber and rubber and the ununiformity in thickness of the outer layer, which belong to the processing defect, were the main reasons of the cracking. The alternating stress and environment caused aging, which accelerated the cracking of the rubber expansion joint.