The mixed grain structure and mechanical property anisotropy of AZ40 magnesium alloy bar with a diameter of 160 mm manufactured by ＂multi-direction forging（MDF） ＋ extrusion ＋ online cooling＂ technique were investigated by optical microscopy（OM）, scanning electron microscopy（SEM）, X-ray diffraction macro-texture measurement and room temperature（RT） tensile test. The results show that mixed grain structure is caused by the micro-segregation of Al in semi-continuous casting ingot. Homogenization of（380 °C, 8 h） ＋（410 °C, 12 h） cannot totally eliminate such micro-segregation. During MDF and extrusion, the dendrite interiors with 3%-4% Al（mass fraction） transform to fine grain zones, yet the dendrite edges with about 6% Al transform to coarse grain zones. XRD macro-textures of the outer, R/2 and center show typical fiber texture characteristics and the intensity of [0001]//Ra D orientation in the outer（11.245） is about twice as big as those in the R/2（6.026） and center（6.979）. The as-extruded AZ40 magnesium alloy bar has high elongation（A） and moderate ultimate tensile strength（Rm） in both extrusion direction（ED） and radius direction（Ra D）, i.e., A of 19%-25% and Rm of 256-264 MPa; however, yield strength（Rp0.2） shows anisotropy and heterogeneity, i.e., 103 MPa in Ra D, 137 MPa in ED-C（the center） and 161 MPa in ED-O（the outer）, which are mainly caused by the texture.（155 °C, 7 h） ＋（170 °C, 24 h） aging has no influence on strength and elongation of AZ40 magnesium bar.
