The yield surface and the deformation mechanism of the extruded AZ31 magnesium alloy rod at room temperature were studied. The uniaxial tensile, compressive and pure shear tests with the simultaneous observation of the microstructure were carried out for specimens sampled along different directions in an extruded rod. In the plane-stress condition within the plane perpendicular to extrusion, the yield surface almost fits the Tresca's criterion for isotropic materials, because of the activation of the basal slip or {10-12} twinning, regardless of the loading direction. The deformation mechanism with higher critical resolved shear stress than those of basal slip or {10-12} twinning is driven by the tensile loading parallel to extrusion, due to the texture with the direction of its c-axis perpendicular to tensile direction. Therefore the tensile yield stress for this direction increase more than compression or shear, and the twinning pattern at the yield for tensile loading does not appear.
