Ab initio all-electron Dirac-Fock-Breit, and Hartree-Fock calculations are reported for UF6, assuming the experimental octahedral geometry, The spin-orbit (S-O) splitting is calculated for all the ground state levels of UF6 and the calculated S-O splittings for the 4a, 5d, and 4f levels are in excellent agreement with those reported experimentally by Martensson et al. [J. Chem. Phys. 80, 5456 (1984)]. The magnetic part of the Breit interaction for UF6 is calculated via perturbation theory as 42 hartrees (1145 eV). Our Hartree-Fock, Dirac-Fock, and Dirac-Fock-Breit wave functions predict UF6 to be bound with dissociation energy of 13.71, 23.53, and 23.27 ev, respectively. Relativistic effects lead to about 70% increment in the predicted dissociation energy of UF6. Our calculations show that the relativistic effects are so large for UF6 that it is imperative to treat them using Dirac’s fully relativistic equation.