This work aims to analyse the effetcts of antimony as a chemical refiner in a hypoeutectic Al-11wt.%Si alloy. The alloy was unidirectionally solidified under unsteady-state conditions, and had its microstructure analysed by measuring the dendrite arm spacings. The results were compared to the unmodified alloy, which proved that Sb addition caused not only higher cooling rates but also the decrease of primary dendritic spacing. Mechanical tests were also performed, and the modified alloy showed an improvement in Vickers microhardness and ultimate tensile strength, preserving the ductility.