Cavitation behaviors related to ferrous primary crystals have been investigated at a temperature of 653 K and a strain rate of 10−3/s for Al-4.5%Mg-0.05%Fe and Al-4.5%Mg-0.2%Fe alloys which have a grain size of 50;Lm. The alloys constantly exhibited a large elongation-to-failure above 300% at the temperature of 653 K and strain rate of 10−3/s. Cavitation was increased by increasing the iron content. Most cavities were nucleated at the interface between the ferrous primary crystal and matrix and elongated parallel to the tensile direction. The experimental critical diameter of the primary crystal, above which cavity is nucleated, was 1.5 µm at the grain boundary and 0.5µm at grain interior, which were very close to double the critical diffusion length.