The in situ X-ray diffraction (XRD) was used during uniaxial tensile loading to study the lattice response and deformation behavior of a2 U-5.8 wt% Nb (U5.8Nb) alloy. Diffraction patterns were recorded at incremental macroscopic strains until fracture of the sample, and the XRD data were analyzed by Rietveld method and single-peak fitting. The changes regulation of lattice parameter, diffraction peak intensity and interplanar spacing with applied stress during deformation were analyzed. It is found that the elastic deformation and the twinning deformation (reorientation) coexist in the initial stage of the first yield platform. The lattice response suggests that the remarkable anisotropy of elastic limit in various directions should be the genesis of double yield curve.