This paper establishes the relationship between the ductility and the pore size for an aluminum casting on the basis of 30 tensile tests on round bars and 30 simple shear tests on butterfly specimens. Pores on each fracture surface are identified and characterized in terms of the projected area. The study reveals that the tensile fracture strain decreases with the area of the largest pore in an approximately linear way while for the shear case a power function gives a more reasonable fitting. The shear ductility is less sensitive to the size of pores than the tensile ductility. The correlations are better described using the area of the largest pore than using the total area of all of the pores. The goodness-of-fit test indicates that the pore size closely follows the log-normal probability distribution rather than either the normal or the Weibull distribution. Meanwhile, the generalized extreme value function is applied to determine the limiting distribution of the maxima of the pore size, which turns out be the reversed Weibull distribution (Type III) for both the round bars and the butterfly specimens in terms of calibration and physical consideration.