The fracture mechanics behaviour of high-nickel austenitic compacted graphite cast iron was studied and the effects of graphite morphology, alloying elements and specimen thickness on the mechanical properties, plane stress fracture toughness, and fatigue crack growth rate were evaluated. It was found that the graphite morphology, i.e. the percentage of compacted graphite present, was the major determinant of all properties of the materials investigated. The irons with a greater amount of compacted graphite (the balance was nodular graphite in austenitic matrix) resulted in lower tensile strength, yield strength, elongation and Kc fracture toughness but higher crack-growth index values (poorer crack-growth resistance). For 25 mm thick specimens, Kc values of the austenitic compacted graphite cast irons in this study were in the range of 58–64 MPa m1/2. This is higher than ferritic/pearlitic ductile iron of 43–53 Mpa m1/2, and is compatible to Ni-resist austenitic ductile iron of 64.1 Mpa m1/2. The addition of cobalt not only contributed to slightly higher values of mechanical properties, but also higher plane stress fracture toughness and better crack growth resistance. Optical microscopy, scanning electron microscopy and X-ray diffraction techniques were applied to correlate the microstructural features to the properties attained.