Measurements of internal friction and Young's modulus between −210° and 700°C have been carried out at frequencies 130–185 kHz on U4O9single crystals. Two pronounced internal friction peaks were observed, one at 60° and the other at −175°C. The peak at the higher temperature seems to be due to a phase transition of U4O9. The factors affecting the peak suggest that it is not caused by a relaxation process. It is suggested that some cooperative movements of interstitial oxygen ions in some lattice planes cause such a transition. The peak at the lower temperature is due to a relaxation effect. A rough calculation of the activation energy gave 0.034 eV. In addition, the results of the electrical conductivity measurements at low temperatures (25°–−160°C) gave the activation energy of 0.057 eV, in an approximate agreement with the one derived from the internal friction measurements. This indicates that the relaxation process involves activated charge transfer between U5+and U4+ions (hopping motion of electronic holes). It is tentatively proposed that stress‐induced ordering of a defect complex involving U5+ion and some oxygen ion(s), which produces a ⟨111⟩ elastic distortion in the lattice, gives rise to the relaxation effect.