Using density functional theory, the structural parameters of α-U and bcc phases were obtained separately, in reasonable agreement with previous experimental and theoretical results. The enthalpy-pressure curve showed that the phase transition pressure was approximately 111 GPa. The structural behavior of γ-U under extreme loading conditions was exhibited by the ideal strength calculations. In addition, by analyzing the energy-strain relationship we found that the strain corresponding to the shear modulus c’ could decrease the total energy of U-Nb (6.25 at.%) alloy, indicating the instability of this structure.