The fracture toughness of ductile cast irons having substantially different microstructures due to Sn addition was studied. The effect of the notch tip diameter on the fracture toughness was also studied. The conditional fracture toughness, K_Q was found to decrease by approximately 50 to 60% with Sn addition. Considering general mechanical properties and fracture toughness, it was concluded that tin addition for strengthening ductile cast iron should be restricted within 0.1%.