<p>Ultrasonic vibration (USV) treatment has been applied on AZ41 magnesium alloy at the various pouring temperatures during the solidification process. The results demonstrated that without the application of USV treatment, the primary <i>α</i>-Mg grains are a coarse dendritic morphology with heterogeneous distribution of <i>β</i>-Mg₁₇Al₁₂ intermetallic particles within the interdendritic regions. The USV treatment has been applied at varying pouring temperatures, leading to significant grain refinement of nearly fine equiaxed grains of the primary <i>α</i>-Mg phases as the pouring temperature exceeds up to 650&#xa0;°C with nearly homogenous distribution of <i>β</i>-Mg₁₇Al₁₂ intermetallic particles along grain boundaries. Further increasing in pouring temperatures leads to increase again in the mean size of <i>α</i>-Mg grains. The optimal pouring temperature at which USV treatment was applied is 650&#xa0;°C. Moreover, with the application of USV treatment, the mechanical properties including hardness, tensile strength as well as elongation% of the examined AZ41 magnesium alloy have been significantly improved. Grain refinement mechanism of microstructure is also explained.</p>