Rolled sheets of AZ31 Mg alloys were subjected to tensile test at room temperature and elevated temperatures. The occurrence of grain-boundary sliding (GBS) at room temperature was demonstrated by the displacement of submicron lines drawn across grain boundaries of deformed samples. Surface relief of deformed samples was measured by use of a scanning laser microscope. GBS strain was calculated from the measured surface step height at grain boundaries. The ratio of GBS strain to total strain was 8% at room temperature and 27% at 523 K. GBS at room temperature was considered to be induced by anisotropic plastic deformation by basal dislocation slip and to contribute to homogeneous deformation.