The microstructures and mechanical properties of a sector ductile iron casting with a wall thickness of 500 mm were investigated. Results show that spheroidal graphite is obtained in ductile iron, and chunky graphite is avoided because of chills used during solidification. Besides, the matrix structure is nearly a full ferrite structure at the edge of casting while some pearlite forms at the center of casting. The competing effect between graphite and matrix structure keeps the strength relatively inert to the change in solidification time or cooling rate, but these two parameters both impair elongation of ductile iron as well. The samples with full ferrite matrix exhibit a typical ductile fracture mode while those with some pearlite lead to a mixed ductile-brittle fracture mode. Graphite internal fracture or graphite/matrix interfacial cracking are two crack initiation sites.