A bending device used for SEM-EBSD systems has been developed by the authors, which enables one to intermittently observe changes in microstructures induced by deformation in the same viewing area. Alternate compression-tension loading was applied to a sheet specimen of AZ31B magnesium alloy by bending the specimen. Twins were formed by compression straining. They remained after un-loading. Applying tensile straining lead to the disappearance of all the twins. Most of twins were those of {10\\bar12} types. Anisotropy in mechanical properties of magnesium alloys would be affected by this twinning and de-twinning behaviors.