We investigated the influence of varying amplitudes at constant frequency on the grain size and mechanical properties of magnesium alloy AZ 91. Our experimental setup consists of an ultrasound generator connected to a steel gravity die by screw fitting. Magnesium alloy AZ 91 has been melted in a closed-argon-flooded steel crucible and poured into the die while ultrasound was generated. Cooling curves were recorded during the whole solidification process. The grain size and the mechanical properties were determined and analyzed. As a result, we achieved reduction in grain size as well as an increase in tensile strengths for amplitudes of up to 40% of the maximum amplitude. Yield strengths could be minimally improved in all experiments. The values of elongation and hardness (Brinell) could also be improved for nearly all investigated amplitudes. In summary it is possible to achieve both: grain refinement and increase in tensile strength, hardness and elongation.