A series of fracture toughness experiments were carried out involving sub-zero-chilled (using liquid nitrogen) cast iron containing 1.5% Cu, and chromium contents ranging from 0.0%-0.2%. By using copper chills of different thicknesses, the effect on fracture toughness of varying the chill rate was also examined. The fracture toughness tests were carried out using three-point bend specimens, each with a chevron notch, as per ASTM E 399-1974 standards. It was found that fracture toughness is highly dependent on the location on the casting from where the test specimens are taken and also on the chromium content of the material. Chill thickness, however, does not significantly affect the fracture toughness of the material, There was found to be an approximately linear relationship between fracture toughness and pearlite content, in which fracture toughness increases as pearlite content decreases and vice versa.