The mechanical properties of electromagnetically stirred billet of Al alloy during continuous casting such as a tensile, impact, and fatigue property was examined with respect to a size and roundness of primary α phase to examine the influence of the globularization and refining of primary α phase on the dynamic mechanical property. The billet was continuously cast in a casting speed of 100 to 600 mm/min during the electromagnetic stirring with a magnetic flux density of 700 Gauss and then was heat-treated. All tensile properties were enhanced with decreasing the size of primary α crystal and the ultimate tensile strength (UTS), yield strength, and elongation was obtained 320MPa, 235MPa, and 17.8%, respectively, at a size of primary α phase of 77 um. The fatigue life at fracture was largely improved by 37% from 1.25×105 to 1.7×105 cycles and the fracture toughness was obtained approximately 7.25 joules as a maximum value at a minimum size of primary α phase on the present experiment condition. Therefore, it indicates that all mechanical properties can be improved at the higher casting speed owing to the fine microstructure of a primary α phase and also eutectic phase due to the higher cooling rate.