In this study, the irreversible expansion of TATB cylinder is investigated. No evident variation of the lattice parameters is observed on TATB crystal. The density of TATB powder decreases by only about 0.02% after it suffered from thermal cycling process at the range from -54 C to 74 C, while the density of TATB cylinder decreases by about 1.0%. It is suggested that the density variation of TATB powder has little contribution to the density decrease of TATB cylinder. Therefore, the increasing interstices between TATB powder originated from the thermal cycling should be responsible to the irreversible expansion of TATB cylinder.