The structure and bond nature for the ground state of the UF5 monomer have been investigated by means of relativistic and nonrelativistic discrete-variational (DV) X alpha molecular orbital (MO) methods. It is found that the nonrelativistic calculation provides only less than half of the U-F bond overlap population obtained by the relativistic calculation for the monomer with the C-4 nu, geometry which was determined experimentally. Comparison of the U-F bond overlap population and valence level structures between the C-4 nu, and D-3h geometries indicates that the stability of the D-3h monomer is almost equal to that of the C-4 nu, one. Furthermore, it is confirmed that an intermediate structure (C-2 nu) between the two geometries shows similar values of effective charge, orbital, and bond overlap populations. This suggests that the energy barrier between the D-3h and C-4 nu geometries is very small and the geometry of the UF5 monomer is fluxional between the C-4 nu and D-3h symmetries. This is consistent with the previous results reported by the Hartree-Fock method with relativistic effective core potentials [Wadt, W. R.; Hay, P. J. J. Am. Chem. Sec. 1979, 101, 5198]. The flexibility of the UF5 structure originates from the fact that the U 5f atomic orbitals, which play a major role in the U-F bonding interactions, spread with wide angular distributions of the electrons in hybridization with the U 6d atomic orbital.