Magnesium alloys are becoming increasingly popular in industrial applications due to their low density and high specific strength. The improvements sought in the formability at ambient temperature and fracture resistance of magnesium alloy AZ31B can be attained through the use of Severe Plastic Deformation (SPD). In order to investigate the deformation response of this material, in situ three-point bending test was performed on a sample extracted from a plate subjected to SPD by constrained groove pressing (CGP). High-energy synchrotron X-ray diffraction was used to examine the lattice strains across the sample. It revealed that significant tension-compression asymmetry is observed for some grain groups. Twinning deformation mode is observed under compressive loading differs from other modes. Additionally, it has been shown that plastic deformation proceeds more readily within grains which experience loading parallel to the basal plane. This is in contrast to the grains whose c-axis is aligned with the loading direction. Stress-strain curves were obtained from XRD and they agree well with results from mechanical test.