U-5.7Nb alloy specimens were artificially aged by isothermal treatments at 190 °C for 1 to 16 h. X-ray diffraction data were collected and discussed. Aging-induced changes in the structure, including lattice parameters, lattice strains and residual niobium in solution, were given. The results show that spinodal decomposition takes place in the U-5.7Nb alloy after aging at 190 °C for 2 h, and further aging causes formation of γ0 phase